How to digitize a graph with a log scale
On a log axis equal distances are equal ratios, so a value read by eye and a ruler is quickly a long way out. Let the axis be read as what it is.
How a log axis is read
The axis is recognised as logarithmic from its tick labels (1, 10, 100, or 10 with a raised power), on either axis or both.
If the labels do not fit one scale, the chart is refused rather than fitted. No reading is better than a wrong one called exact.
Steps
- Drop in the graph.
- Check a ring or two against the gridlines.
- If the axis was read wrongly, choose Set the axis yourself, click two places on it, type their values and tick This is a log axis. Every value on that axis is read again.
Good to know
When a PDF holds the chart as a drawing (most figures in papers do), the values are read from the drawing itself. Nothing is estimated from pixels, so a point is as precise as the file is.
Small, crowded figures are the hard case for pictures: when a plot is only a couple of hundred pixels high, a pixel is already a visible error. Use the PDF when you have it, or a larger screenshot.
Questions
Does it handle log-log plots?
Yes. Each axis is judged by itself, so one or both can be logarithmic.