How to digitize a graph with two y axes
A chart with a second axis on the right is two charts sharing an x axis. Read against the wrong one, a series is off by whatever the two scales differ by.
How each series finds its axis
Where the axis labels are printed in a series' colour, that settles it.
Where nothing on the chart says, it is worked out from the order the chart was drawn in, and the result tells you so, so that you can check the series marked right axis.
What we measured
- From PDFs: 299 of 300 charts with two y axes read correctly.
- From pictures: 15 of 15 on a fresh set, after 21 of 25 on the first.
Steps
- Drop in the chart.
- In the table, each series on the right axis is marked.
- Download: the workbook says which axis each series was read against.
Questions
What about bars with a line over them?
The bars are read against the left axis and the line against the right, as the chart shows them.