Pairplot support

[TheCedarPrince]

Based off a Slack conversation where I saw that Plots.jl doesn't have native support for pair plotting, I am opening this PR to support the missing functionality!

Currently just a draft. @daschw - I will ping you more when this is ready for review/merging. Right now, is there anything in particular you want me to prioritize with this functionality? Thanks!

[codecov]

Codecov Report

Merging #3255 (a72ba38) into master (6fb0d69) will not change coverage.
The diff coverage is n/a.

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##           master    #3255   +/-   ##
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  Coverage   62.91%   62.91%           
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  Files          25       25           
  Lines        6081     6081           
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  Hits         3826     3826           
  Misses       2255     2255           

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[daschw]

Hi, sorry, apparently I was not fast enough on Slack 😉
I would put the code in src/recipes.jl and implement it as a user recipe. This way users of pairplot can still modify Plots attributes and are not limited to the default values for e.g. ytickfont you set in the function. For inspiration you could have a look at the corrplot recipe in StatsPlots https://github.com/JuliaPlots/StatsPlots.jl/blob/master/src/corrplot.jl.

[TheCedarPrince]

No worries! Probably good that it is here so we have a record of it @daschw. I will be working on this over the next couple days/weeks.