[APM] Change the "Time spent by span type" chart to use latency instead of percentage

[formgeist]

Summary

We should convert the "Time spent by span type" chart from percentage to duration on the y-axis. That way it's easier to relate the distribution between each span type to accumulated latency.

The title should be changed to Average duration by span type

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[smith]

I read an article recently that may be worth considering: Stacked Area Graphs Are Not Your Friend.

[formgeist]

@smith Thanks for sharing the article. I do agree with a lot of the general points on how to use stacked areas, but we currently can't afford some of the alternatives (displaying individual charts for each series). I think perhaps we can experiment with the chart itself if we convert it to duration instead of %. Additionally, we could offer an option to switch between % and duration inside the chart panel allowing the user to see either chart.

[botelastic]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.