remove "arc__" text from legend

[jtorkkel]

At the moment in Node Graph all legends contain text prefix "arc__" like "arc__failed" or "arc_passed".

Describe the solution you'd like
I would like to see legend not including "arc__" test. Grafana own AWS Node Graph and infinity datasource does not show "arc__" text when graph visualized.

[hoptical]

Currently, there is no specification about "arc__" legends in the node graph documentation.
This will force us to search in the node graph source files like this one. But I couldn't find anything there too. It just handles the color and its mode, not the legend.