Update docs for fortify

[bkkkk]

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[yutannihilation]

I agree fortify()'s doc needs to be updated. But, maybe we need to discuss about the current status of fortify() first? I think fortify() is still useful as part of the ggplot2's API; it's just not useful for general use of converting any object to data.frame.

[bkkkk]

@yutannihilation this was brought up in issue #3717, I thought I’d get a legs up on resolving part of it. I agree that the whole fortify family in ggplot2 would benefit from a look-over. The current documentation says that it might get deprecated, I figured until this happens it’s worth pointing people to a “better” solution.

How should we proceed? Maybe create a separate issue from #3717 to hold this discussion?

[yutannihilation]

Hmm, sorry I think I need some time to answer...

Basically, there are roughly 3 types of objects that fortify() converts.

1. model object (I think this is why the first argument is named model)
2. sp object
3. data.frame-alikes (e.g. grouped_df)

I think category 1 is superseded by broom::augment() (for lm object) and broom::tidy() (for multcomp objects), and category 2 is by geom_sf(). What I said "is still useful as part of the ggplot2's API" above is category 3.

#3717 is only about sp objects, so we need another issue for broader discussion. Let me open a new one later.