Geom.segment update

[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

• updates Geom.segment (fixes How to make lines with Julia using Gadfly (Facility location problem)? #1337)
• Geom.segment supports one-length aesthetics e.g. linestyle=[:dash], color=[colorant"red"]
• adds gallery example of the type of plot in How to make lines with Julia using Gadfly (Facility location problem)? #1337 (see p2 below)

Examples

a = range(0, stop=7π/4, length=8)+ 0.2*randn(8)
D2 = [DataFrame(x2=x, y2=x, x=x.+sin.(a)/r, y=x.+r*cos.(a), 
        ls=rand(["A","A","B"], 8)) for (x,r) in zip([1,-1], [0.4,0.3])]

p1 = plot(D2[1], x=[1], y=[1], xend=:x, yend=:y, Geom.segment, color=[colorant"forestgreen"], linestyle=[:dash])
p2 = plot(vcat(D2...),
    x=:x, y=:y, xend=:x2, yend=:y2, color=:x2, linestyle=:ls, Geom.point, Geom.segment, 
    Scale.linestyle_discrete(levels=["A","B"]), Scale.color_discrete,
    Theme(key_position=:none, point_size=3.5pt))
hstack(p1, p2)

[codecov-commenter]

Codecov Report

Merging #1465 into master will increase coverage by 0.03%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master    #1465      +/-   ##
==========================================
+ Coverage   89.41%   89.44%   +0.03%     
==========================================
  Files          39       39              
  Lines        4402     4416      +14     
==========================================
+ Hits         3936     3950      +14     
  Misses        466      466              

Impacted Files	Coverage Δ
src/geom/segment.jl	96.00% <96.15%> (+1.40%)	⬆️
src/statistics.jl	96.45% <100.00%> (ø)
src/ticks.jl	90.71% <0.00%> (-0.66%)	⬇️
src/scale/scales.jl	91.95% <0.00%> (+1.14%)	⬆️

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[bjarthur]

the 1-length aesthetics here and in your other recent PRs is great! what about supporting scalars as well? that is, color=colorant"red". would make sense from the user POV i think, but easy to change the code?

[Mattriks]

See #1430. Please add comments there about aesthetic syntax, so we can work out what's best.