Accessible charts start with a table

A line chart is a good way to show a shape. It is a terrible way to convey a number to somebody using a screen reader, and adding alt text that says “chart showing traffic over time” helps nobody.

The approach that works is boring. Build the table first, with the real numbers in it. Then draw the chart from the same data and mark it as presentational.

This has a side effect worth having. When the underlying table exists, anybody can copy the numbers into a spreadsheet without exporting anything, and the person who asked “what was Tuesday” can just read it.

It also keeps you honest about smoothing. A chart can quietly interpolate. A table cannot, and if the figures look wrong in the table then the chart was flattering them.