plots and metrics: update new targets rules

[pared]

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Addresses #1800
Related to iterative/dvc#4590

[jorgeorpinel]

Done here! Thanks @pared

[jorgeorpinel]

Please merge when you deem appropriate @pared (and feel free to include iterative/dvc#4620 (review) here).

[shcheklein]

@pared looks good to me! let's resolve conflicts and ship it :)

[jorgeorpinel]

From #1809 (review):

I think if explain in the plots concept itself that plots are not only dvc.yaml based people won't expect dvc.yaml to be present in the first.

And from #1809 (review):

It's better to specify at the top level that plots files are by definition could be anywhere.

I don't think we've done this yet though @shcheklein @pared. That would imply a change in the plots cmd ref index, no?

[pared]

@jorgeorpinel I agree that we need to update the description of the commands, but we need to finish up here first.
I think we are pretty clear that targets don't have to be in dvc.yaml. Its mentioned both in plots/show and plots/diff. Do you think we should stress it even more?

[jorgeorpinel]

I think we are pretty clear that targets don't have to be in dvc.yaml. Its mentioned both in plots/show and plots/diff. Do you think we should stress it even more?

True. But I thought the idea was that we wouldn't need to mention it in those or at least not stress it much as long as it was clearly expressed in the index that DVC can work with any valid plots file whether or not in dvc.yaml.

[jorgeorpinel]

That said I agree we can merge this with the notes so far, although the index still needs a small update so I unlinked this from #1800

Also the note in plots show could probably be in a different place: not sure why it's part of the first paragraph, seems disconnected with the other sentences there (see #1809 (review))

[jorgeorpinel]

Wait... What about https://dvc.org/doc/command-reference/plots/modify?

Pleas see my last comment ^

[pared]

@jorgeorpinel well, in this case, plot need to be defined in dvc.yaml - if we want to modify it, we need a place to store the modification information. So, while we can attempt to show and diff pure data with templates that we already have, modifying requires us to save some info. I suppose we could support that, but as of now, we don't do that. Trying to modify non-plot will result in error.
ERROR: Unable to find DVC-file with output '{path}' We should probably make it more informative. I will create an issue for that.

[jorgeorpinel]

while we can attempt to show and diff pure data with templates that we already have, modifying requires us to save some info. I suppose we could support that

Ah, makes sense @pared . I'm not suggesting that we support that BTW, I just didn't think it through.

Thanks

p.s. I see you created iterative/dvc/issues/4741 about the error message 👍