Profile page contributions chart is inaccessible

[aardrian]

I also filed this via the official method using https://github.com/contact.

The existing GitHub contribution chart that appears on user profile pages is inaccessible in multiple ways:

• it is unusable by keyboard
• it relies on color and contrast to convey information
• it relies on a mouse to convey information
• it does not adapt to Windows High Contrast Mode
• it does not honor browser text size settings
• it does not work with any screen reader / browser combination

I built an example accessible version, which you may completely ignore, as part of a response to another re-imagining of the chart and thought it might be a good idea to file a bug report as well.

The example: http://adrianroselli.com/2018/02/github-contributions-chart.html