Two series, one axis each

Fri, 08 Sep 2023 00:00:00 +0000

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.

The crossing is a design decision

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.

What a research notebook is for

Fri, 21 Apr 2023 00:00:00 +0000

Laboratory scientists learn early that an experiment not written down did not happen. The bound notebook, with numbered pages and dated entries in ink, is one of the oldest instruments of experimental science, and it survives because it does something no other record does: it captures what the experimenter knew and intended at the moment of acting.

Computational work has inherited the experiments but not, in most places, the notebook. A modern analysis leaves behind a repository full of code, a directory full of outputs, and perhaps a commit history. It is easy to believe that this is a complete record. It is a record of what was done. It is almost never a record of why.

Essays on DataLeaf

Two series, one axis each

The crossing is a design decision

What a research notebook is for