Add legend for average task runtime graph

[simond]

Description

When clicking on a task in the grid view, a newly-created chart that shows the duration of the tasks shows two reference lines. It is not clear what these reference lines mean and there should be a legend that describes what they are. Is one the average duration and the other the median duration?

I think it would make sense to have a legend either below or beside the graph to describe what the reference lines are.

Use case/motivation

The motivation is to allow users to understand what the lines on the chart represent.

Related issues

No response

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[eladkal]

Yes I mentioned it also in #38214 (comment)
cc @tirkarthi

[tirkarthi]

Yes, it was decided to keep mean and not average. The blue line is mean task run duration. The gray line is mean queued duration.