RFC: Subplot labeling. Fixes #998

[apalugniok]

#998
I opted for ann = (:top_left, :auto) instead of ann = :top as it makes the processing easier.

using Plots
plot(rand(10,8), ann = (:top_left, :auto), layout = grid(2,2))

plot(rand(10,8), 
ann = [([:top_left, :top_right], ["test", "(b)"]) (:top_center, :auto) ([5, 9], 0.5, ["middle", "stuff"])],
layout = grid(2,2))

Note:

• Polar plot annotations don't behave well. The interpretation of annotation coordinates differs between backends.
• 3D plots also don't work.

[daschw]

Really great work @apalugniok !

[daschw]

I'd use versions without underscores (:topleft, ...) here to be consistent with the position options for the legend attribute.
The Plots way would probably be to allow both versions, as well as :t, :tl, :tr, ... for annotations and legendpositions. This could be achieved by introducing a _positionAliases Dict and do some preprocessing in args.jl.

[apalugniok]

Forgot to mention Plotly cannot deal with annotations when multiple subplots are present.

plot(rand(10,8), ann = (:bottom_left, :auto), layout = grid(2,2))

[daschw]

IMO we can merge this and open an issue for plotly.

[abehersan]

Hi! I just realized this convenience feature existed from the discourse posts! Is it documented in the API guide at all and I just missed it entirely? I could not find anything...

[BeastyBlacksmith]

I don't think it is documented