Flamegraph: Bring back diff scale legend

[Rperry2174]

In the old flamegraph we had a scale that helped users understand that the diff view is slightly inverted (red is positive numbers and green is negative) numbers which is counter-intuitive.

https://flamegraph.com/share/793cd196-9835-11ee-9a0d-f2c25703e557

I was doing a demo with someone from the community today and they were really caught up on this. We should bring this back so that users understand that

• "green === negative / decrease (numerically) === good / better"
• "red === postive (numerically) / increase === bad / worse"

Same goes for the colorblind color pallette -- without a legend it's unclear what the colors mean here

cc @darrenjaneczek