plots diff: Show previous revisions for new plots

[dberenbaum]

If I add a new plot, dvc plots diff should be able to try to render previous revisions even if the plots were not configured in that revision.

For example, let's say I have plots data in train.csv for multiple revisions and then add a plot in dvc.yaml:

stages:
  train:
    cmd: train
    plots:
    - train.csv:
        x: epoch
        y: loss
        title: stage_plots

dvc plots diff shows only the workspace, but it should also show the HEAD revision.

[dberenbaum]

@pared Is a feature request or a bug? Any thoughts on priority?

[pared]

It's a feature request, I wanted to include it in the current state of #7477 but it's already hanging for too long, let's do it right after it gets merged.

[dberenbaum]

Let's figure out priority later. If it hasn't been supported until now with few complaints, no need to rush when we have other priorities.

[pared]

For anyone having a problem with this, a workaround, for now, would be to create new commit that contains desired plot definitions, before starting to experiment.

[mattseddon]

Should work if the revision with the required plot is the first rev in the diff call.