Mesh chart legend

[timchown]

The new grafana mesh views don't include a legend describing the colour meaning.

At least for https://ps-mesh.perf.ja.net/grafana/d/aa15bb77-25d6-53d1-b3c7-bf51b6417afa/jisc-uk-tests?orgId=1, which we took from the default mesh template.

It would be good to add this.

[ChristopherWalker]

Further work would be to have a control to allow users to change it (with a default set by the mesh operator).

[timchown]

"Users" being whom though? The mesh owner, or anyone viewing it? And how might the changes be made?