3. 2. Doubly linked discussion

To situate conversation around the visualization, we created a technique called doubly linked discussion. The method begins with an “independent” discussion interface in which users can attach comments to particular states (or views) of a visualization. Comments are shown on the right side of the Web page and grouped into linear discussion threads (Figure 1e). Each comment shows the thread topic, comment text, the author’s full name, and the time at which the comment was authored. Clicking on a comment takes the visualization to a bookmarked state representing the view seen by the comment’s author.

Users can add comments either by starting a new thread or posting a reply to an existing thread. When a “New Comment” or “Reply” link is clicked, a text editor appears at the site where the comment will be inserted and the graphical annotation tools (discussed next) become active. Upon submission, the comment text and any annotations are sent to the server and the comment listing is updated.

The interface described above is based on links from the commentary into the visualization. Our system also provides links in the other direction: from the visualization into the discussion. As a user changes parameters and views in the visualization, they may serendipitously happen upon a view that another person has already commented on. When this occurs, the relevant comments will automatically appear in the right-hand pane. Our intuition was that this “doubly linked” discussion interface, which combines aspects of independent and embedded discussion, would facilitate grounding and enable the visualization itself to become a social place.

3. 3. Pointing via graphical annotation

In real-time collocated collaboration, participants commonly use both speech and gesture, particularly pointing, ^{11, 20} to refer to objects and direct conversation. For asynchronous collaboration, graphical annotations can play a similar communicative role. We hypothesized that graphical annotations would be important both for pointing behavior and playful commentary. To add a pictorial element to a comment or point to a feature of interest, authors can use drawing tools (Figure 1b) to annotate the commented view. These tools allow free-form ink, lines, arrows, shapes, and text to be drawn over the visualization view. The tools are similar to presentation tools such as Microsoft PowerPoint and are intended to leverage users’ familiarity with such systems.

Comments with annotations are indicated by the presence of a small shape logo to the left of the author’s name in the comment listing (see Figure 1e). When the mouse hovers over an annotated comment, the comment region highlights in yellow and a hand cursor appears. Subsequently clicking the region causes the annotation to be shown and the highlighting to darken and become permanent. Clicking the comment again (or clicking a different comment) will remove the current annotation and highlighting.

We refer to this approach as geometric annotation, which operates like an “acetate layer” over the visualization, in contrast to data-aware annotations directly associated with the

underlying data. We chose to implement a free-form annotation mechanism so that we could first study pointing behaviors in an unconstrained medium. Aside from the freedom of expression it affords, geometric annotation also has a technical advantage: it allows reuse of the identical annotation system across visualizations, easing implementation and preserving a consistent user experience.

3. 4. collecting and linking views

In data analysis it is common to make comparisons between different ways of looking at data. Furthermore, storytelling has been suggested to play an important role in social usage of visualizations, as discussed by Viégas et al. Drawing comparisons and telling stories both require

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the ability to embed multiple view bookmarks into a single comment.

To support such multiview comments and narratives, we created a “bookmark trail” widget. The bookmark trail functions something like a shopping cart: as a user navigates through the site, he or she can click a special “Add View” link to add the current view to a graphical list of bookmarks (Figure 1c). Bookmarks from any number of visualizations can be added to a trail. A trail may be named and saved, making it accessible to others.

The bookmark trail widget also functions as a short-term storage mechanism when making a comment that includes links to multiple views. Dragging a thumbnail from the bookmark trail and dropping it onto the text area create a hyperlink to the bookmarked view; users can then directly edit or delete the link text within the text editor. When the mouse hovers over the link text, a tooltip thumbnail of the linked view is shown.

3. 5. awareness and social navigation

Social navigation¹⁵ leverages usage history to provide additional navigation options within an information space. Our initial system supports social navigation through comment listings and user profile pages that display recent activity. Comment listings provide a searchable and sortable collection of all comments made within the system, and can be filtered to focus on a single visualization (see Figure 3). Comment listing pages include the text and a thumbnail image of the visualization state for each comment. Hovering over the thumbnail yields a tooltip with a larger image. Clicking a comment link takes the user to the state of the visualization where the comment was made, displaying any annotations included with the comment. The author’s name links to the author’s profile page, which includes their five most recent comment threads and five most recently saved bookmark trails. The view also notes the number of comments made on a thread since the user’s last comment, allowing users to monitor the activity of discussions to which they contribute.

Although more elaborate social navigation mechanisms are possible, we wanted to observe system usage with just these basic options. We were particularly interested in observing the potential interplay between data-driven exploration and social navigation. By allowing discussions to be retrieved unobtrusively while a user explores the data,