https://dataleaf-642346.gitlab.io/tags/visualization/Recent content in Visualization on DataLeafHugoen-USFri, 08 Sep 2023 00:00:00 +0000https://dataleaf-642346.gitlab.io/posts/dual-axes/Fri, 08 Sep 2023 00:00:00 +0000https://dataleaf-642346.gitlab.io/posts/dual-axes/<p>Few chart forms are as tempting, or as quietly misleading, as the line chart with two vertical axes. The motivation is honest: two quantities with different units seem to move together, and putting them on one chart invites the reader to compare them. The trouble is that the comparison the chart makes is not in the data. It is in the choice of axes.</p> <div class="dataleaf-heading"><h2 id="the-crossing-is-a-design-decision">The crossing is a design decision</h2> <a class="dataleaf-heading-anchor" href="#the-crossing-is-a-design-decision" aria-label="Permalink to The crossing is a design decision" data-heading-anchor data-heading-copy="Copy link to The crossing is a design decision" data-heading-copied="Copied link to The crossing is a design decision.">#</a> </div> <p>A line chart maps values to vertical positions. With one axis, the mapping is shared, and every visual relationship between the lines is a relationship between the numbers: one line above another means one value exceeds the other. With two axes, each series gets its own mapping, chosen independently. Where the lines sit relative to each other, whether they cross, and how steep they look are all consequences of two arbitrary pairs of numbers: the limits of the right-hand axis.</p>