[APM] Can't deselect/hide average series for transaction error rate chart

[dgieselaar]

The legend for "average" in the transaction error rate chart is disabled, meaning the series can't be hidden. For other charts, this is possible, so I would consider this a bug.

[elasticmachine]

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

[cauemarcondes]

I remember @formgeist requested it, can you confirm it?

[dgieselaar]

Ah that probably makes sense when I look at the breakdown chart.

[formgeist]

@dgieselaar @cauemarcondes Indeed, I requested that we disable it because there's only a single series. Doesn't really make sense that you would be able to disable it and get an empty chart. It's a little custom, but hope that's OK in this case.