Change the formatting of the chart legend

Problem to solve

Currently our charts legends reside below the each chart, following a format of Label Avg Max split into multiple columns, e.g.

Grafana has a very nice format, where it shows the legends in a single column, even adding some shades of colors to help distinguish odd from even rows.

We should probably use that format if possible, but there are some caveats.

Do we want to create our own legend component? This would allow us to iterate a little bit faster as we have the control of the code from the get-go but it could also slow us down by introducing bugs and or unexpected regressions
Do we want to contribute to echarts upstream? A little bit slower as we're not familiar with the echarts codebase, but this would make a good case to give back to them as they've been really helpful to us in the past.