Allow multiple plots from one plot file

Consider a mock dvc.yaml such as:

stages:
    A:
        cmd: ...
        plots:
            - metrics.csv:
                x: x
                y: y1
            - metrics.csv:
                x: x
                y: y2

When using dvc plots show, DVC will only show one (the last) plot. The fix is to split the one metrics file into multiple metrics files with the same x but different y variables. This feels pretty inconvenient for metrics that are related in their independent variable, and so can be stored in the same table and file.

On the surface I don’t see any reason why a specified plot artifact can only be used for one plot.

[daavoo]

By “combining” we have been usually referring to get multiple (y) columns from either the same file or multiple files, which seems reasonable given how we usually recommend formatting the data in linear plots.

However, I think that the user in https://github.com/iterative/studio-support/issues/23#issuecomment-929932069 showcases a new scenario that we haven’t discussed so far in https://github.com/iterative/dvc/discussions/5980#discussioncomment-1026072 . It is basically using the values of a third column (z) to group the values of a single column (y) and the template looks very clean.

IMO this use case could be easily covered on the DVC side by extending the vega template and adding a new placeholder DVC_METRIC_COLOR_LABEL and we could encourage this format as a quick workaround for combining y values. On the DVCLive side, we could add some logic to facilitate the optional usage of this format (i.e. adding a new stage column for grouping based on the metric name).

[shcheklein]

@dberenbaum should we/can we prioritize this?