=Paper= {{Paper |id=Vol-2778/paper3 |storemode=property |title=Towards Designing a Tool For Understanding Proofs in Ontologies through Combined Node-Link Diagrams |pdfUrl=https://ceur-ws.org/Vol-2778/paper3.pdf |volume=Vol-2778 |authors=Tamara Flemisch,Ricardo Langner,Christian Alrabbaa,Raimund Dachselt |dblpUrl=https://dblp.org/rec/conf/semweb/FlemischLAD20 }} ==Towards Designing a Tool For Understanding Proofs in Ontologies through Combined Node-Link Diagrams== https://ceur-ws.org/Vol-2778/paper3.pdf
 Towards Designing a Tool For Understanding Proofs in
  Ontologies through Combined Node-Link Diagrams

              Tamara Flemisch1 , Ricardo Langner1 , Christian Alrabbaa2 , and
                                   Raimund Dachselt1
                1
                    Interactive Media Lab Dresden, Technische Universität Dresden
                2
                    Theoretical Computer Science, Technische Universität Dresden
                             Nöthnitzer Str. 46, 01187 Dresden, Germany
                           firstname.lastname@tu-dresden.de



         Abstract. As the creation of large ontologies is a difficult and error-prone pro-
         cess, we think it is particularly relevant to develop new visual and interactive tools
         that support exploring ontologies as well as finding and resolving defects, such
         as undesired logical entailments. Exhibiting formal proofs for the undesired en-
         tailments can help in understanding how a defect happens, and linking the steps
         in the proof to the ontology can help in determining how the defect can be fixed.
         We present an interface that visualizes proofs and the corresponding ontology in
         form of side-by-side node-link diagrams. Building on linked brushing, users ben-
         efit from a strong interplay between these views, which allows for discovering
         and understanding defects. Besides traditional desktop workplaces, our interac-
         tion design also considers the use of touch input enabled by interactive displays.
         As part of an iterative design process, we developed an initial web-based pro-
         totype implementation and gathered feedback from an interview with domain
         experts. With this ongoing research and development, we aim to further inves-
         tigate the potential and general utility of interactive visualizations for ontology
         engineering.3


1      Introduction and Related Work

Creating and maintaining large ontologies is an error-prone process. The large size and
the intricate relation of the knowledge represented in ontologies makes the task of on-
tology debugging difficult [12]. Examples for errors that appear in ontologies are unsat-
isfiable concepts or undesired entailments, which we refer to as defects in the following.
However, tools exist to aid users in finding incorrect or undesired entailments. Explor-
ing formal proofs for the undesired entailments can help in understanding the origin of
a defect. Furthermore, connecting the proof of the undesired statement to the ontology
can aid in determining how a defect can be fixed. This can be achieved by computing
diagnoses for the defect and showing their impact on the whole ontology. We think, it
is highly relevant to support this process of exploring ontologies as well as finding and
resolving defects by developing new visual and interactive tools.
 3
     Copyright © 2020 for this paper by its authors. Use permitted under Creative Commons Li-
     cense Attribution 4.0 International (CC BY 4.0).


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Towards Designing a Tool For Understanding Proofs in Ontologies


     Visualization and interactive methods have been applied to a multitude of different
 application domains. Especially linking and combining multiple visualizations, e.g.,
 multiple coordinated views (MCVs), is a common approach to solve complex prob-
 lems [27]. It has been used to address challenges inherent to visualizations, such as
 filtering and querying [5], collaborative exploration through meta-visualizations [32],
 and visualizing change in dynamic networks [18]. Some of these techniques were then
 applied to domain-specific problems, such as exploring multivariate data [17], observ-
 ing the sharing of pictures in social networks [18], and visualizing genealogy [26] and
 biology data [21]. Multiple visualizations have also been investigated in non-traditional
 setups, such as large, interactive displays [17, 32] and mobile devices [16, 28]. Espe-
 cially in the domain of visualizing ontologies, plenty of tools exist and were collected
 and compared in a comprehensive survey by Dudás et al. [6]. An example is Matent-
 zoglu et al.’s [23] Inference Inspector which helps in understanding ontologies by vi-
 sualizing entailment set changes. Among the variety of different techniques, such as
 node-link visualizations, radial layouts, and treemaps, are specific examples, such as
 indented lists used in Protégé [15], Jambalaya [31], OWLViz4 , and KC-Viz [24]. They
 use a vast selection of interaction techniques, such as filtering, keeping a history, high-
 lighting, and edge bundling. However, most of the existing tools have issues [6]:

   – They often have a UI, visualization and interaction design that leaves room for
     improvement regarding current technologies
   – They only focus on class hierarchies of ontologies and are not tailored towards
     understanding proofs
   – They focus on a text based representation instead of a visualization of the content

 Interaction plays a huge role when it comes to exploring and understanding data [19,
 8] in general. More specifically, previous work has looked at interaction techniques
 for exploring tree visualizations [20] and node-link diagrams [5, 3, 22]. Furthermore,
 natural interaction, e.g., through pen and touch input, has also been successfully applied
 to network visualizations to aid with exploring [7, 18, 14] as well as editing [9].
      We present an interface that visualizes proofs and the corresponding ontology in
 form of side-by-side node-link diagrams for ontology debugging of unwanted conse-
 quences. Whereas the view of the ontology, which displays its modular structure, has
 its origin at the top of the view, the proof tree shows the root node, i.e., the final con-
 clusion of the proof, at the bottom. We build on common interaction techniques, such
 as linked brushing [2, 27] and overview+detail [4, 25], to create a strong interplay be-
 tween these components to support the users in discovering and understanding defects.
 In addition to traditional desktop workplaces, we also consider the use of touch input
 enabled by interactive displays and propose adaptations to leverage the benefits of nat-
 ural interaction.
      In this work, we describe our tool based on the process we went through. First,
 we describe the use cases on which we based our design. After elaborating more on
 the detailed visualization and interaction concepts, we explain the technical details of
 the implementation. Finally, we discuss adaptations based on what we learned from
  4
      https://github.com/protegeproject/owlviz


                                            29
Towards Designing a Tool For Understanding Proofs in Ontologies


 our prototype and the feedback we gathered from the domain experts for our initial
 prototype and.


 2     Application Interface
 Our application is a prototypical web application for ontology debugging of unwanted
 consequences. It consists of two main components: the Proof Component which offers
 an interactive visualization for explaining defects by using proofs and the Ontology
 Component which visualizes the ontology in its modular structure and shows diagnoses
 of defects and their impact on the ontology.
     In this section, we start by describing the use cases we derived from several ana-
 lytical interviews with expert users. We then explain how these use cases inspired our
 visualization and interaction design for traditional desktop setups.

 2.1   Use Cases
 We conducted several analysis sessions experts for Description Logic ontologies [1].
 During the first sessions, we made use of paper-based and whiteboard sketches to un-
 derstand the domain, the data, and the problem with existing tools. In later sessions,
 we also incorporated their data, i.e., a graph representing the ontology in its modular
 structure and formal proofs as trees, into exiting graph visualization tools. Our goal was
 to discover issues with current visualizations, to identify challenges for visualizing both
 types of data at the same time, and to explore possible scenarios. For all sessions inde-
 pendent of the technologies used, we interviewed the domain experts and derived the
 following use cases for visualizing proofs along with their respective ontologies with
 regard to ontology debugging.

 Comparing and Contrasting Ontologies are often subject to change and a collabora-
 tive effort of a whole community. This results in bugs potentially being introduced by
 accident from one version to the next. Therefore, it can be beneficial to compare the
 proofs for undesired entailments in addition to the ontology among different versions.
 This visual comparison can help to spot differences, identify changes, and potentially
 even help to understand the origin of these changes [10].

 Understanding and Exploring Since ontologies are large constructs that are often hard
 to grasp for novice users, one use case is the explaining of ontologies. We believe that
 exploring a proof, i.e., a designated part of an ontology, can help novices in understand-
 ing the inner workings and the structure of ontologies. Furthermore, breaking down a
 graph into chunks that are easier to understand at once, might help in exploring the
 overall network.

 Showing and Explaining Analogous to the previous use case, presenting and exploring
 a proof might also help when explaining an ontology’s structure to other people. The
 proof’s layered and stepwise structure helps in breaking down a larger problem into
 little chunks. It can further aid in explaining how logical reasoning works by example
 which can be leveraged for educational purposes.


                                            30
Towards Designing a Tool For Understanding Proofs in Ontologies


 Repairing and Fixing The most important use case for our application scenario is re-
 pairing and fixing parts of an ontology. After having found the undesired entailment,
 the user aims to fix it by iteratively navigating a proof tree and identifying the origin of
 the bug. Additionally, the connection between ontology and proof might help the user
 to decide how to fix a particular bug by computing diagnoses.

     After carefully evaluating the possible use cases for visualizing proofs and their cor-
 responding ontology, we decided to mostly focus on the primary use case of Repairing
 and Fixing in addition to a secondary use case of Understanding and Exploring. How-
 ever, our proposed interface does not exclusively support one use case, it still supports
 other use cases in parts. In summary, our main aim was to assist users in achieving the
 following tasks:
   – Understanding entailments through proof exploration
   – Understanding the interaction of axioms in the ontology
   – In case of unwanted entailments, understand how to fix the ontology and the effect
     of axioms removal on the modular structure of the ontology

 2.2    Visualization, Interaction Design, and Features
 Before designing our application, we decided on some requirements that shaped our
 design process:
   – The application is predominantly used in a traditional desktop environment.
   – It should be possible to use the application across distributed devices, i.e., having
     the two visualizations on different devices.
   – Our main goal is to seamlessly integrate both components into one interactive tool.
    To make it easier for novices to understand, we are using a commonly used example
 ontology, the Pizza Ontology in a modified way5 in our examples and figures. The
 modification introduces the defect SpicyIceCream � Pizza.

 The Proof Component The proof component consists mostly of the proof tree of a
 chosen entailment with the root node being the proven entailment, i.e., the conclusion
 (see Figure 1). In our example, the proof shows the unsatisfiability of SpicyIceCream.
 The leaf nodes represent a so called justification for this entailment, i.e., statements
 that are part of the ontology and justify the entailment as being true. The levels in-
 between show the logical reasoning from the justifications to the entailment. By ex-
 ploring the proof, the user may find other defects that lie within the proof, e.g., that
 SpicyIceCream � Pizza. The Proof Component is the center of our application and
 serves as a starting point for exploring the ontology and finding the origin of a de-
 fect. For showing the proofs, we use a node link diagram that puts the leaf nodes on top
 and the root node at the bottom of the view. This node-link encoding emphasizes the
 connection among nodes, their depth within the tree, and its topological structure [30],
 i.e., the various paths that lead to the final conclusion. We decided to create an axis-
 oriented tree layout since it is very common and most users will be familiar with it [29].
  5
      Available at https://lat.inf.tu-dresden.de/Evonne/PizzaOntology/


                                            31
Towards Designing a Tool For Understanding Proofs in Ontologies




                           Fig. 1. Screenshot of our tool showing Proof Component


 Because a proof can be reasonably large and hard to understand at a glance, we provide
 interactive techniques for navigating the tree and making a large proof easier to under-
 stand. The button at the top (cf. Figure 1) loads a chosen proof from a GraphML file
 and displays it within the component. Clicking on one of the nodes, i.e., axioms of the
 proof, reveals iconic buttons that allows the user to access navigation and communica-
 tion functionalities (cf. Figure 2).
     The navigation buttons and the switch for the Stepwise Mode allow for a stepwise
 exploration of the proof in both directions, top-down and bottom-up. With top-down
 and bottom-up, we consider the tree’s structure and not the position of the nodes, i.e.,
 bottom is the leaf node level whereas top is the root node.


           Generate and                                                       Hide all prev. inferences
           Show diagnoses                                                     Show all prev. inferences

       Communication buttons                                                  Navigation buttons


      Show justification
      of the corresp. proof                                                   Show the next inference


                               Fig. 2. Buttons associated with axioms in proofs

     In the top-down approach, the exploration starts with showing only the final conclu-
 sion. Previous inferences can be revealed step-by-step. This allows the user to be fully
 in control of the exploration and focus on the paths they are interested in. This way,
 the next node, i.e., inference, is only revealed once the user decides that the visible part
 has been understood. In the bottom-up approach, i.e., starting from the leaf nodes, the
 justification, users can collapse certain parts of the proof as understood which thereby


                                                     32
Towards Designing a Tool For Understanding Proofs in Ontologies


 decreases the size of the proof and allows the user to increase the focus by omitting
 information. By providing both approaches, we allow the users to traverse the proof
 according to their preferences.
     In case of very hard to comprehend parts of the proof, the user can take a sub-proof
 and isolate it from the remaining tree by using the ”delink” button on the edge to the
 following node (cf. Figure 3). This provides a detail view of all inferences that lead to
 the chosen link. By clicking on the red node on the bottom, the user is taken back to the
 original view of the proof.




 Fig. 3. Clicking on the delink icon next to the link in the left image isolates the sub-proof starting
 from this connection, which can be seen in the right image.


     Besides their large size, proofs can also be hard to understand because of the rules
 that are applied in each step. The rule might be puzzling because the user might not be
 familiar with it or because the axiom itself is too long to easily spot the application of
 the rule. To aid the user in comprehending these rules and inference steps, we designed
 a tooltip that can be invoked by clicking on a particular rule (cf. Figure 4). The tooltip
 provides the following contents: The abstract rule using substitute variables for the
 concepts, the current instance of the rule below the abstract rule, and a color encoding
 that explains how the abstract rule was applied.

 The Ontology Component The ontology component computes the diagnoses for the
 defects and visualizes their impact through the ontology’s modular structure. Its role
 is to put the proof in context and give an overview of the whole ontology. As shown
 in Figure 5, the component consists of two parts, the modular structure as a node-link
 diagram and the part that shows possible diagnoses for a defect. Regarding the layout of
 the node-link diagram, users can either use a pre-computed force-directed layout [13],
 arrange the nodes by dragging to create a custom layout which can then be saved, or load
 a layout. We provide two types of labels for the nodes. The default label lists all axioms
 that are contained in the node whereas the other option labels the node with the signature
 of axioms in this node, such as Toppings, while respecting the dependency between
 the nodes. A collapsed side menu shows the computed diagnoses for a certain axiom.


                                                  33
Towards Designing a Tool For Understanding Proofs in Ontologies




        Fig. 4. Explanation of an instance of the Intersection Composition rule.




                Fig. 5. Screenshot of our tool showing the Ontology Component


 These diagnoses are grouped into collapsed panels based on their size to add structure to
 the otherwise convoluted list. Hovering over a diagnosis highlights the corresponding
 axioms in the node-link diagram by changing their color. Using linked brushing [2]
 allows us to explore the relations between the different types of data [27]. It also changes
 the color of all nodes that contain these axioms and their predecessors which shows the
 impact of the diagnosis on the ontology.



 Interplay Between the Components We provide two techniques that connect the two
 main components of our tool, which are both triggered in the Proof Component. How-
 ever, the effects are shown in the Ontology Component. The first technique is high-
 lighting diagnoses. A user can select any node, i.e., axiom, in the Proof Component by
 clicking on it and choosing the button displaying a magic wand (cf. Figure 2) to generate
 all subset-minimal diagnoses for the selected axiom. This computes a list of diagnoses
 which is then shown in the Ontology Component in the side menu. The second tech-
 nique is highlighting a justification of an axiom. By clicking the button displaying the
 magnifying glass, the corresponding node and axiom in the Ontology Component can
 be highlighted through color change. This is an especially important feature for repair-
 ing ontologies as it helps users to see the axioms in the ontology that lead to the selected
 one in the currently considered proof.


                                             34
Towards Designing a Tool For Understanding Proofs in Ontologies


 3     Prototype Implementation
 Our prototype was implemented as a web application using Express6 and Node.js7 on
 the server side. The Node Express server supports multiple clients, which can either dis-
 play the Proof Component or the Ontology Component. Thereby, we allow the user to
 distribute the views according to their preferences on different displays or even on dif-
 ferent devices in the same network. We made this choice early on to ensure that we have
 the possibility to extend the usage of our tool to multi-device environments (MDEs). On
 the client side, we use D3.js8 for the visualizations and the CSS framework Milligram9 .
 Furthermore, the tool supports GraphML files to load the data and to load and store
 the layouts. Diagnoses and highlighting is calculated in separate Java applications. To
 enable the clients to properly communicate among themselves and with the server, we
 use Socket.IO10 .


 4     Adaptations and Expert Interview
 In this section, we elaborate on the insights we gained from implementing the prototype
 and from an interview with domain experts.

 4.1   Adaptions for Natural Interaction on Interactive Surfaces
 From the beginning of the design, we had the idea of extending our application to also
 work with natural interaction, such as pen and touch input. Touch devices have become
 ubiquitous and are frequently used for analysis tasks [19, 11]. Therefore, we made
 specific design choices to ensure that the tool is extendable in the future, such as having
 a client-server architecture that allows for a multi-device setup. Regarding our interface,
 we created interactive mockups using Adobe XD for a future design iteration of two
 main parts: (1) the design of the nodes and (2) the navigation of the proof.
     As mentioned in the previous sections, we want to provide the user with a variable
 top-down and bottom-up approach. To improve our current design, we were inspired by
 natural and fluid interaction [8] for visualizations. In the fully collapsed state of a proof
 (cf. Figure 6), only the leaf and the root node are visible. All other nodes are hidden
 behind a Magic Rule. By pulling a node away from the Magic Rule, the next step that
 was hidden behind the Magic Rule is revealed. Pushing a node back towards the Magic
 Rule hides the last step of the proof. This approach can be used until there is no Magic
 Rule left as well as to create several Magic Rules within one, possibly large, proof.
     As shown in Figure 7, the new node design is less minimal than the old one and
 includes all functionality within the node instead of using the space around the node. It
 has two states: the collapsed, i.e., default, and the expanded state. The collapsed state
 only provides the most important features, i.e., expanding and collapsing the child or
  6
    http://expressjs.com/
  7
    https://nodejs.org/en/
  8
    https://d3js.org/
  9
    https://milligram.io/
 10
    https://socket.io/


                                             35
Towards Designing a Tool For Understanding Proofs in Ontologies




 Fig. 6. Left: A proof with only leaf and root nodes being visible, Right: After pulling on axiom
 a1 to reveal another step of the proof.




 Fig. 7. Left: A node of an axiom in its collapsed state, Right: The node in its expanded state
 showing additional functionalities


 parent node. The expanded state, however, allows access to all features and additionally
 explains features that are otherwise only accessible through an icon, e.g., expanding
 and collapsing.
     Even though the designs are not yet implemented in the prototype, we aim to provide
 the users with a more natural and thus intuitive interaction for working with a proof and
 an ontology by improving the overall node design and the navigation techniques.

 4.2   Expert Interview
 As part of our iterative design process, we conducted an interview with three domain
 experts working daily with Description Logics ontologies. These were not the same
 experts that took part in our initial analysis sessions (cf. subsection 2.1). We showed
 them our prototype and explained how we imagined the workflow with the application.
 Additionally, after receiving their feedback on the current prototype, we also presented
 the design adaptations (cf. subsection 4.1). The comments were mostly about the theo-
 retical background of the modular structure we use for the Ontology Component since


                                               36
Towards Designing a Tool For Understanding Proofs in Ontologies


 it is a rather uncommon way of representing an ontology. This structure was not imme-
 diately clear to the users and could only be fully understood after an explanation and
 discussion with one of the authors. This, however, is conflicting with our goal of being
 easy to use for domain experts. Therefore, we consider switching to a more traditional
 concept-based approach for the Ontology Component. The experts mentioned a lot of
 minor design and implementation related issues, like improving icons or increasing the
 font size. Most of the minor comments are already addressed by our design revisions.
 Especially, our new version of navigating the proof and the introduction of the Magic
 Rule received very favorable comments. In general, the experts were convinced by the
 concept itself but expressed concerns about the modular structure of the ontology and
 whether it would be reasonably easy to understand and thereby helpful.


 5      Conclusion
 We presented an interface that visualizes proofs and the corresponding ontology as side-
 by-side node-link diagrams to facilitate ontology debugging of unwanted entailments.
 We provided an overview of our visualization and interaction design including the Proof
 Component and the Ontology Component and some technical details. Furthermore, we
 elaborated on further adaptations for natural interaction and interactive surfaces and the
 feedback we gained from an expert interview. In the future, we aim to incorporate our
 adaptions into our prototype and conduct a larger user study to further investigate the
 potential and utility of interactive visualizations for ontology engineering.


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