html graphs: allow merging data between graphs

[timo]

sometimes two adjacent graphs have enough in common that it would be interesting to have the lines from both graphs show up in one (for example, the benchmark varieties "something" and "something_native"). Especially the fact that the scales are usually different between those graphs makes this particularly helpful.

I imagine a button next to the graph that would first select a source graph, then a target graph to merge and on the third click would remove the data again or something like that