Plot charts at interval level?

[david]

You could start by calculating pace/HR (or something similar, I only do cycling) for each interval and then maybe combine that into one number for the activity. It looks like some people have already done some of this work (under the activity timeline chart do Fields → Custom and search):

Then you could plot that over time on the /fitness and /compare pages.

Here is an overview of how you can extend Intervals.icu:

Extending Intervals.icu

This post pulls together all the ways you can extend Intervals.icu. Some require a little Javascript knowledge and others just picking a value from a drop down. Intervals.icu also has an API.