Add a % diff to the compare view graphs

[Nooshu]

Is it possible to add a % difference to the bar charts in the compare view graph?

For example this test from the BBC shows a 14% improvement from migrating from JPEG to WebP. It would be great if this figure was calculated automatically, I guess this could be applied only when a diff is detected for certain metrics.

[github-actions]

We're in the process of cleaning up issues on this project in order to ensure we're able to stay on top of high priority bugs and feature requests. As a part of this process, we're automatically closing any issues that have had no activity within the last two years, including this one, since the codebase has changed dramatically in that time. If you feel this is still relevant, please file a new issue using the relevant issue template so we can get it prioritized. Thanks!