A chart's y axis should (almost) always start with zero. Right?
Cole Nussbaumer (storytellingwithdata.com) and Jon Schwabish (policyviz.com) recently had a conversation about this very topic and posted it at storytellingwithdata.com, along with some example charts. In summary, Cole and Jon agreed that column/bar charts should always start their y-axis with zero. Because our eyes focus on the height of that bar, a non-zero axis distorts the relationship between columns. Savvy viewers of the chart may also be more skeptical about the truth of our visualization when they notice that the axis doesn’t start at zero.
But what if you really need to focus on small but meaningful differences between data points? Line graphs might be ok here depending on the context, because the focus in a line graph is on the relative position of the points in space rather than the height of the bars. But the relative positions can still be overstated by a non-zero axis.
One solution Cole and Jon (and I!) like is to present 2 charts side-by-side: one that has a zero-starting y axis to show the context of the data, and another that does not start at zero, but instead zooms into the chart to show the variation within a smaller range.
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