plots: smart legend labeling

[dberenbaum]

Background:

• Initial support for flexible plots #7477 (comment)
• plots: flexible dvc.yaml spec #7086 (comment)

After #7477, the legend can sometimes be too long. Removing the common parts of the legend would make it easier to read.

Also, in #7086, the y-axis isn't read from the column name even though the column name is the same in both files.

[dberenbaum]

@pared It looks like this is already addressed in #7477?

[pared]

Yes, it should be working now.

[pared]

Though I still believe we should make a change in the plots templates to address @daavoo's idea

[dberenbaum]

Closing in favor of iterative/dvc-render#83

[dberenbaum]

Reopening since I think there are still improvements to make here. For example:

Some possible improvements:

1. In the title, including the directory path might be useful, but I think we can omit the dvc.yaml filename. What other file could the configuration be coming from?
2. In the legend, training/plots/metrics are the same for all entries, so can we omit it?
3. The full rev is cut off but is actually training/plots/metrics/train/acc.tsv::train/acc. train/acc is redundant. One way to shorten it would be to omit the paths unless the field names conflict. Another would be to shorten the name in dvclive from train/acc to acc since train is already in the path (or log all dvclive metrics to one file).