Feature request: facets

[fredcallaway]

It would be really nice if StatsPlots could support faceting (i.e. making subplots by grouping by a variable in a table). I think it would make sense to use the seaborn syntax, e.g.

@df school density(:MAch, row = :Sx, col = :Sector)

to create four subplots laid out in a grid with subplot titles like "Sx = Girls, Sector = Catholic".

Easy construction of facets is in my opinion one of the two biggest things missing from the julia plotting ecosystem (as far as I know) when compared to Python (seaborn) and R (ggplot). (If you're curious, the other thing is a simple syntax for plotting summary statistics).

It seems to me that this would be a straightforward extension of the existing grouping machinery. But the fact that this hasn't been implemented already makes me think I must be missing some nuance that makes this tricky... I'd be happy to work on a pull request if this is something you'd be likely to accept.

[mkborregaard]

We already have

@df school density(:MAch, group = (:Sx, :Sector), layout = (2,2), link = :all)

Can you see a good way of improving on this?

A little more powerful, and probably soon going to be plotting-package-independent, is here: http://juliaplots.org/AlgebraOfGraphics.jl/stable/generated/tutorial/#Layout

[fredcallaway]

Ah excellent! This is already most of the way there. As the obi-wan meme that you edited out (but I was fortunate enough to see in the email notification) suggested, I think this might be an un(der)-documented feature.

Since you asked, I see two ways this could be improved:

1. Make an option to label the groups with subplot titles rather than a legend—currently, the legend becomes difficult to read if you also have three groups (one being the color)
2. Make it possible to infer the layout by the number of unique values in each group. I think a natural way to accomplish this would be with the syntax

@df school density(:MAch, layout = (:Sx, :Sector), link = :all)

But I can imagine that both of these would be somewhat difficult to integrate into the existing grouping machinery. And thanks for the pointer to AoG!