For brevity, only visual encoding and mapping features utilised in this research are discussed in this section. Information regarding other mapping properties and tag cloud layout algorithms can be found in the following technical documents:
Deaker and Churcher [2011]; Deaker et al. [2011] (note that these documents describe the basic procedure which hasn’t changed much, whereas the graphical interface, numbers of options, and features have changed significantly through the course of this research).
The mapping selection interface can be seen in Figure 4.3 on page 54. In the analysis of visual properties available in tag clouds (Chapter 3), order, size,
Figure 4.2: A tag cloud generated by Taggle depicting a subproject of ActiveMQ with metrics derived from CKJM. Visual mapping interface is on the left.
colour and transparency were deemed the most appropriate visual variables for data mapping purposes. The order in which they are displayed on the central mapping pane is in order of perceived usefulness and importance (with tag order being the most useful and transparency the least useful). By default, only the minimum mappings essential for display (tag and tag order) are turned on.
The user is supported in the choice of appropriate mappings for the tag cloud by a data summary screen shown in a resizeable pane at the top of the GUI — for an example see Figure 4.4 on the following page. Each field in the data file is shown along with data type (text, numeric or categorical) and the range of values in the category. The data summary panel was included by request from a heuristic evaluation (see Chapter 7).
4.3.1 Categorical and continuous data
Data types displayed in the data summary screen are determined programmati-cally. Fields are labelled categorical when the value ranges correspond to a certain maximum distinct number of values (the actual number is controlled via a set-ting). It is limited to a particular number of distinct values to match the number of colour chips displayed in the legend — categorical variables are used predomi-nantly in display colour mappings (§4.3.5). Providing a method for distinguishing
(a) Data summary panel toggled on (b) Data summary panel toggled off Figure 4.3: Mapping selection interface: the data summary panel can be toggled by the user as desired to save valuable screen space
Figure 4.4: Closeup of an example data summary panel
these two types of data was included by request from the heuristic evaluation, where it was determined to be useful to display categorical data using a unique
colour for each category.
4.3.2 Tag
Visual variable: tag length The user is free to select any field to map to the tag text, including numeric types. The tag lengths for each tag will differ depending on the number of characters in the tag text. To minimise the possible effects on perception long textual identifiers may have, see§4.4.1for a description of the application of filters.
Visual variable: font family The font face dropdown box is pre-populated with fonts that are installed on the machine running Taggle. Because reading ease is improved when presented with familiar fonts [pg 106, chap 11:7 Health and Services, 2006], Arial, Helvetica and Times New Roman (or the equivalents depending on the operating system and installed word processing software) are presented for selection at the top of the dropdown box, separated by a divider (see Figure 4.5).
Figure 4.5: Prompting selection of familiar fonts.
4.3.3 Order
Visual variable: order If the order visual property is mapped to a field con-taining textual data, ordering of the tags is determined alphabetically, otherwise
tags are ranked by their numerical value. The selected ordering can be reversed with the “reverse order” checkbox and updating the tag cloud. The user can select an appropriate layout from typewriter, spiral or force directed (defaulting to the simplest layout, typewriter).
4.3.4 Size
Visual variable: font size Research has indicated slower reading performance for smaller font size [pg 107, chap 11:8 Health and Services, 2006], so there are constraints on the minimum font size to be set to no less than 9pt. The maximum font size is constrained to 250pt: by default this is set to a much smaller 35pt to allow for a smaller screen and canvas size (canvas size can be adjusted from the settings tab). The data mapped to the selected font sizes can be reversed with the “reverse size” checkbox.
4.3.5 Colour and transparency
Visual variable: background colour The heuristic evaluation and controlled experiments conducted in this research determined that the use of background colour was useful for visual search with colour mappings, as well as dataset fea-tures such as multiple word tags. Background colour has also been found to support more accurate estimation of relevant tags [Waldner et al.,2013]. There-fore tag display via background colour is the default setting in Taggle.
The user may select from a choice of colour range (between user defined colours) or colour buckets (a set palette of colours) — see Figure 4.6 on the following page. When the user chooses a data field to map to colour, colour range or colour bucket is automatically selected depending on whether the data field is categorical data. Categorical data is best displayed through a unique set of colours preselected from a colour palette (the colour palettes can be user defined through external string property files, although default palettes are provided.) For example, in Figure 4.7 on the next page a dataset about agile projects is displayed in a tag cloud. With the aim of finding what actors are related to the agile stories with the highest time estimates, order/size are mapped to estimate and colour is mapped to actor. The user can determine stories with the greatest
estimated workload corresponding to tags coloured dark grey and brown, and find details via a colour legend mouse hover or tag label mouse hover (actors
‘Preferred customer’ or ‘Bookstore customer’).
Continuous data is best displayed through a colour range. The user has the option of choosing a colour gradient between two selected colours, or colours which appear on the colour wheel between two selected colours (see Figure4.8 on the following page. Colour models are complicated and won’t be discussed in full detail). Like other mappings, the user also has the option to reverse the selected mappings so high values can be shown with the ‘from’ colour and low values can be shown with the ‘to’ colour.
Figure 4.6: Colour mapping interface
(a) Mouse over the legend (b) Mouse over a tag Figure 4.7: Displaying categorical data with colour buckets
(a) Colour gradient between red and blue (b) Colour wheel between red and blue
(c) A colour wheel
Figure 4.8: Displaying continuous data with colour ranges
Visual variable: font colour The user also has the choice of displaying a more conventional looking tag cloud, with colour mapped to the font rather than the background (see Figure 4.9 on the next page).
Visual variable: transparency The user may map data fields to a visual property which is familiarly used in tag clouds online — transparency — see Figure 4.10 on the following page. This can be applied with or without colour.