  Cartograms are map-based data visualizations in which the area of each map
region is proportional to an associated numeric data value (e.g., population or
gross domestic product). A cartogram is called contiguous if it conforms to
this area principle while also keeping neighboring regions connected. Because
of their distorted appearance, contiguous cartograms have been criticized as
difficult to read. Some authors have suggested that cartograms may be more
legible if they are accompanied by interactive features (e.g., animations,
linked brushing, or infotips). We conducted an experiment to evaluate this
claim. Participants had to perform visual analysis tasks with interactive and
noninteractive contiguous cartograms. The task types covered various aspects of
cartogram readability, ranging from elementary lookup tasks to synoptic tasks
(i.e., tasks in which participants had to summarize high-level differences
between two cartograms). Elementary tasks were carried out equally well with
and without interactivity. Synoptic tasks, by contrast, were more difficult
without interactive features. With access to interactivity, however, most
participants answered even synoptic questions correctly. In a subsequent
survey, participants rated the interactive features as "easy to use" and
"helpful." Our study suggests that interactivity has the potential to make
contiguous cartograms accessible even for those readers who are unfamiliar with
interactive computer graphics or do not have a prior affinity to working with
maps. Among the interactive features, animations had the strongest positive
effect, so we recommend them as a minimum of interactivity when contiguous
cartograms are displayed on a computer screen.
