Horizontal or vertical ribbon

[Mattriks]

• I've updated the documentation to reflect these changes
• 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

• Add "vertical" orientation for Geom.ribbon (closes Geom.ribbon bounded left and right <- Feature Request #1452)
• Remove testscript colored_ribbon.jl (that test is included in testscripts/ribbon.jl)

Example

X = [cos.(0:0.1:20) sin.(0:0.1:20)]
D= [DataFrame(x=0:0.1:20, y=X[:,j], ymin=X[:,j].-0.5, ymax=X[:,j].+0.5, f="$f") for (j,f) in enumerate(["cos","sin"])]

p1 = plot(vcat(D...), x=:x, y=:y, ymin=:ymin, ymax=:ymax, color=:f, Geom.line, Geom.ribbon, alpha=[0.6])
p2 = plot(vcat(D...), x=:y, y=:x, xmin=:ymin, xmax=:ymax, color=:f, Geom.path, Geom.ribbon)
hstack(p1, p2)

[codecov-io]

Codecov Report

Merging #1513 (6bae95a) into master (eef6074) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1513      +/-   ##
==========================================
+ Coverage   89.72%   89.82%   +0.09%     
==========================================
  Files          39       39              
  Lines        4613     4637      +24     
==========================================
+ Hits         4139     4165      +26     
+ Misses        474      472       -2     

Impacted Files	Coverage Δ
src/geom/ribbon.jl	100.00% <100.00%> (ø)
src/scale.jl	95.63% <0.00%> (+0.48%)	⬆️
src/aesthetics.jl	79.69% <0.00%> (+0.75%)	⬆️

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[humphreylee]

Excellent. Thank you.