[APM] Consider hiding the legend values from the charts since individual values are shown in tooltips

[formgeist]

Summary

Considering that the individual plot values are already showing in the chart tooltip, having the legend values feel superfluous. Additionally, when not hovering the chart, the legend values represent the last bucket not the overall average of the selected time range. The charts team have an open issue to address the average value in the legend item #84478

Proposal

Do away with the legend value and only use it to show the legend name for the series.

Considerations

Hiding the legend values will be against the default usage of the charts, so there might be some inconsistencies to see in other areas of Observability. In most other cases we are also employing the chart tooltip to show the values. One interesting implementation we use in the Observability overview page is having a stat component in the panel to show the average values for the selected time range in a bigger bolder visual.

[elasticmachine]

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

[formgeist]

Closing as we're moving back to design for a decision on this from a unified Observability/Kibana perspective