Multiple SOMs on same axes

Ability to display multiple SOMs on the same axes. It's fine to restrict this to SOMs that come from the same TC, i.e. ones that share the same TC config.

e.g. display SOM 240 and SOM 247 overlaid.

It might be nice to show "corresponding" SOMs (having the same {job, build, tc_config, som_config}) in the same colour by default? (Or perhaps to have an option so that that's configurable.)
