Stacked guides

[Mattriks]

• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR

• enables stacked guides (Roadmap Roadmap #1385)
• closes Feature request: map multiple numerical and categorical data to plots #766

Examples

using RDatasets
Dsleep = dataset("ggplot2", "msleep")
Dsleep = dropmissing!(Dsleep[:,[:Vore, :BodyWt, :SleepTotal, :SleepRem]])
Dsleep.SleepTime = Dsleep.SleepTotal .> 8
p1 = plot(Dsleep, x=:BodyWt, y=:SleepRem, Geom.point, color=:Vore, shape=:SleepTime,
    Scale.x_log10, Guide.colorkey(),
    Guide.shapekey(title="Sleep\n(hours) ", labels=[">8","≤8"]),
    Theme(point_size=5pt, key_swatch_color="slategrey",         
        point_shapes=[Shape.utriangle, Shape.dtriangle],
        discrete_highlight_color=identity, alphas=[0.1])
)

The number of key columns is automated. In this example I reduce the size of the testscript plot,
so the keys gain 2 columns. The max number of columns is controlled by Theme(key_max_columns=4).

p2 = evalfile(dirname(pathof(Gadfly))*"/../test/testscripts/point_color_shape_size.jl")
draw(PNG(5inch, 3.3inch), p2)

[codecov-io]

Codecov Report

Merging #1423 into master will decrease coverage by 0.18%.
The diff coverage is 75.51%.

@@            Coverage Diff             @@
##           master    #1423      +/-   ##
==========================================
- Coverage   89.57%   89.39%   -0.19%     
==========================================
  Files          39       39              
  Lines        4288     4318      +30     
==========================================
+ Hits         3841     3860      +19     
- Misses        447      458      +11     

Impacted Files	Coverage Δ
src/guide/keys.jl	81.81% <66.66%> (-0.83%)	⬇️
src/guide.jl	88.99% <76.74%> (-1.65%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0e51e2a...3c9ae73. Read the comment docs.

[Mattriks]

This is ready to merge - if there are no comments

[bjarthur]

it's beautiful! does it appear in the gallery, tests, and docs?

[Mattriks]

Yes in testscripts "Guide.shape_key" and "point_color_shape_size.jl". I changed a gallery example Guide.shapekey to show stacked guides (note there are other examples in the gallery with inside guides). Done!