  Reading a visualization is like reading a paragraph. Each sentence is a
comparison: the mean of these is higher than those; this difference is smaller
than that. What determines which comparisons are made first? The viewer's goals
and expertise matter, but the way that values are visually grouped together
within the chart also impacts those comparisons. Research from psychology
suggests that comparisons involve multiple steps. First, the viewer divides the
visualization into a set of units. This might include a single bar or a grouped
set of bars. Then the viewer selects and compares two of these units, perhaps
noting that one pair of bars is longer than another. Viewers might take an
additional third step and perform a second-order comparison, perhaps
determining that the difference between one pair of bars is greater than the
difference between another pair. We create a visual comparison taxonomy that
allows us to develop and test a sequence of hypotheses about which comparisons
people are more likely to make when reading a visualization. We find that
people tend to compare two groups before comparing two individual bars and that
second-order comparisons are rare. Visual cues like spatial proximity and color
can influence which elements are grouped together and selected for comparison,
with spatial proximity being a stronger grouping cue. Interestingly, once the
viewer grouped together and compared a set of bars, regardless of whether the
group is formed by spatial proximity or color similarity, they no longer
consider other possible groupings in their comparisons.
