VC-11 Define styles. In the sketches, some graphic settings exist that do not encode infor- mation and which cannot be controlled by mappings. Examples of such settings that we call styles, are the border- and background colours of objects (Fig. 3.3b) and shadows (Fig. 3.4c). Styles assign a single constant value or a combination of constant values to a set of elements. One of multiple possible values for a graphic attribute is chosen, without necessarily encoding meaning.
VC-12 Benefit from good defaults. Style settings should be based on good defaults. An example for this is the colour of text (dark coloured font on bright backgrounds, bright on dark ones; Fig. 3.2a) or the concrete appearance of directed connectors (e. g., an arrow with a middle large arrow head; Fig. 3.2d). Beyond style settings, also other defaults could be assumed for many sketches. For example, the labelling of graphic objects with the label of the resource they represent is such a setting that could become a mapping default.
3.7
Requirements
Based on our general goal of effective, tailor-made and reusable visualisations of ontological data by end-users, the concrete visualisation cases we identified in the previous section, as well as on experiences other researchers described for visualisation and presentation approaches (Chapter 4), we formulated 15 requirements (R-1 – R-15 ) for our visualisation approach. These requirements partly correspond to the criteria for characterising related work, which are described in Sect. 4.1.2 in more detail. In brackets after each requirement, we point to problems (P-X ), visualisation cases (VC-X ) and other, more general requirements that explain why we laid down the requirement.
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Figure 3.5: Sketch of a tabular representation of amino acids from the Amino Acid Ont. (aa:) using two spatial dimensions to segregate amino acids by aa:hasCharge and aa:hasHydrophobicity. Additionally, mappings from aa:hasSideChainStructure and aa:hasPolarity to shape (Fig. 3.2b) and labelling (Fig. 3.3d) are reused (AA-5).
CHAPTER 3. PROBLEM ANALYSIS
3.7.1
Requirements for Visualisation and Interaction
R-1 Dynamic and value-dependent visual mapping – not only presentation (P-1, VC-2, VC-3) R-2 Variety of graphic relations (P-2, VC-3)
R-3 Interaction with the visualisation (VC-5)
First, we require our approach to support configuration of dynamic and value-dependent visual mappings (R-1), i. e., to allow for actual visualisation as opposed to just structuring data into a document shape or formatting data (font type or background colour). Further, instead of focusing on node-link diagrams (graphs) alone, the approach shall cover a large variety of graphic relations (R-2) such as containment (enclosure), alignment and the separation of graphic objects by a separator. Additionally, alsosimple interactions (R-3) need to be possible to define such as »Highlight all linked nodes on select«.
3.7.2
Requirements for Data Awareness
R-4 Ontology-aware (A/T-Box) (P-7, VC-12)
R-4a Terminological ontology relations (T-Box) supported (P-7) R-5 RDF supported (P-7, P-8)
R-6 Domain agnostic (P-7)
We require the visualisation approach to be aware of the specifics of ontologies (R-4). That means processing and representing RDF on a graph level is not considered sufficient and implies that we need to allow for referencing ontology terms within visualisation settings. We further require that also a specific visualisation of T-Box statements is possible and that these relations can explicitly be referenced in mappings (R-4a). Since all domain ontologies from our case studies are available in RDF, we add the requirement that RDF-based ontologies need to be supported (R-5). Additionally, because we did not restrict ourselves to a certain domain we want to visualise, it is important that our approach is domain agnostic (R-6).
3.7.3
Requirements for Reuse and Composition
R-7 Reusability of the defined mappings (P-3, VC-7)
R-8 Composability of the defined mappings (P-4, VC-7, VC-8) R-9 Explicit mapping definitions (VC-7, R-7, R-8)
We require visualisation settings made for one data set to be reusable for other similar data sets using the same ontological terms (R-7). Visualisation authors should be able to adapt existing settings found on the web to their specific use cases. Related to this, a composition of multiple visualisation settings has to be supported (R-8) – as far as this is possible with respect to the constraints and rules that apply to the composition of graphics, such as
3.7. REQUIREMENTS
syntactic and perceptual interactions, expressiveness and effectiveness12. We require the general
composability of visual mappings and demand that our approach offers the foundation for checking constraints and rules of graphic composition. The problem of actually checking the constraints is something that we do not expect to be completely solved in the scope of this thesis. Our definition of composability includes that we allow for mixing visual paradigms such as node-link and tabular representation. Composability of mappings adds to reusability, since only if users can combine mappings with other existing mappings, they can fully benefit from existing work. A prerequisite for composing and reusing existing mappings is that these mappings have to be made explicit and can be stored and referenced by name (R-9).
3.7.4
Requirements for Variability
R-10 Platform variability (P-3, P-5) R-11 Visual structure variability (P-2)
Since we aim at reusing mappings defined by others, e. g., in a different visualisation tool, we require platform variability (R-10), i. e., it should be possible to vary the graphic platform (e. g., exchange SVG by X3D) and still use the same visualisation settings. Similarly, mappings should be composable and exchangeable independently as far as possible, even when we vary the visual structure or visual paradigm13, e. g., from list to tree structure (R-11).
We already anticipated that we decided to use a declarative approach for defining visual mappings. This was done for reasons of separation of concerns (separating presentation logic from data) and the resulting benefits for reusability [Lie05, Huy07, HB10]. We discuss the advantages and disadvantages of a declarative approach in more detail in the context of the RDFS/OWL Visualisation Language (RVL) in Sect. 7.2.
3.7.5
Requirements for Tooling Support and Guidance
R-12 Domain experts can visualise their data without programming or visualisation skills (P-7, VC-12)
R-13 Visualisation settings configurable with a GUI (P-7, R-12) R-14 Interactions configurable with a GUI (P-7, R-12)
R-15 Guidance for Visual Mapping with a GUI (P-7, R-12)
R-16 Consider complex semantics of an ontology for visual mapping (P-7, R-12)
As described in the introduction to this chapter, our target user group includes domain experts, which may lack programming skills or visualisation expertise or both. Since domain experts should equally be able to visualise their data (R-12), a GUI should be offered for the configuration of visual mapping (R-13) and interaction settings (R-14). To allow not only for configuring some visualisation, but support effective visualisations (e. g., adhering to laws of visual perception), we further require guidance for the visual mapping process via a GUI.
12We come back to rules for graphic composition in Sect. 5.7.3.
CHAPTER 3. PROBLEM ANALYSIS
Beyond the guidance support that existing visualisation design suites offer, we require guidance to consider the complex semantics of ontologies for the visual mapping advices (R-16).
3.7.6
Optional Features and Limitations
O-1 Configuration results instantly shown O-2 Data Filtering
O-3 Guidance for Data Selection O-4 Guidance for View Transformations L-1 No support for editing the visualised data
The features O-1 – O-5 are considered optional with respect to the prototype that has to be built, since they are not in the focus of this thesis. Nevertheless, integrating features such as a mechanism for data filtering (O-2) is essential for a real world scenario. Technologies for creating visual queries or faceted browsing [ODD06] are possible candidates for filtering. Another problem we do not try to solve with this thesis is toinstantly show the result of configuration changes (O-1). While the classical visualisation pipeline model/architecture fails with respect to this feature, the need for instantly reflecting changes has been recognized and approached by [Bul08] and also in the context of visual analytics, which requires fast configuration-feedback cycles [KS12].
Two other optional features concern the extension of guidance to other parts of the visualisa- tion process. While we focus on guidance for configuring visual mappings, guidance could be extended to data selection steps (guidance for data selection, O-3) and to the navigation in the rendered scene (guidance for view transformations, O-4).
Finally, although we design the visualisation language to be well usable by tooling in order to allow for convenient editing of the visual mapping definitions, we do not aim at editing support for the underlying source data (L-1).
Chapter 4
Analysis of the State of the Art
In this chapter we give a detailed analysis of the state of the art in the fields that are touched by this thesis. These fields are approaches to visualisation design, approaches to RDF presentation, the corresponding languages that are used within these approaches, and, finally, approaches to generating user interfaces from models.
Accordingly, the analysis of the state of the art is split into four parts: In a first section, we compare visualisation approaches as a whole, before looking at the languages, that often correspond to one of these approaches, in detail. The comparison of languages is split again into visualisation languages and RDF presentation languages presented in the second and third section of this chapter. For each of these three areas, we first give a brief overview of each system or language we inspected. Then, we precisely define a certain reference criterion and finally compare all items by this criterion, pointing to the most important differences. A tabular overview sums up and completes the comparison. Finally, in a fourth and last part of this state of the art analysis, we need to review approaches of generating interfaces from models. This is due to the fact that we aim at dynamically generating the visualisation system based on several models – the source data ontologies, visualisation ontologies and a concise schema of a visual mapping language (which will be introduced in the following chapters). Since we already know that we have to reference ontology terms in many situations, we also examine what options are at hand for using »traditional« modelling technologies and ontologies in conjunction.
At the end of each section, we briefly conclude the results of the analysis. We see that none of the visualisation approaches examined here fulfills all the requirements put for our approach; neither does an RDF visualisation language exist, which could be reused.
CHAPTER 4. ANALYSIS OF THE STATE OF THE ART