Geom.line update

[Mattriks]

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

This PR:

• updates Geom.line (following Missing and NaN support Compose.jl#397, fixes Weird stacking of layers #1391)
• Geom.line supports one-length arrays e.g. linestyle=[:dash] or color=[colorant"red"] (following Increase support for color aesthetic #1438). This issue has been mentioned e.g. Is there an easy way to pass aesthetics? #818 (comment)

To be done:

• bump compat for Compose.jl (needed for this PR)
• in the Gadfly docs, replace the syntax layer(..., Geom.line, Theme(...)) where possible

Example:

y1 = [0.1, 0.26, missing, 0.5, 0.4, NaN, 0.48, 0.58, 0.83]

plot(x=1:9, y=y1, Geom.line, Geom.point, color=["Item 1"], linestyle=[:dash], size=[3pt],
    layer(x=:1:10, y=rand(10), Geom.line, Geom.point, 
        color=["Item 2"], size=[5pt], shape=[Shape.square]),
    layer(x=:1:10, y=rand(10), color=[colorant"hotpink"], 
        linestyle=[[8pt, 3pt, 2pt, 3pt]], Geom.line))

[codecov-commenter]

Codecov Report

Merging #1459 into master will increase coverage by 0.76%.
The diff coverage is 93.54%.

@@            Coverage Diff             @@
##           master    #1459      +/-   ##
==========================================
+ Coverage   89.40%   90.16%   +0.76%     
==========================================
  Files          39       39              
  Lines        4380     4181     -199     
==========================================
- Hits         3916     3770     -146     
+ Misses        464      411      -53     

Impacted Files	Coverage Δ
src/Gadfly.jl	77.75% <0.00%> (+1.74%)	⬆️
src/scale/scales.jl	91.66% <90.32%> (+2.95%)	⬆️
src/geom/line.jl	100.00% <100.00%> (ø)
src/scale.jl	97.56% <100.00%> (-0.05%)	⬇️
src/ticks.jl	90.32% <0.00%> (-1.05%)	⬇️
src/mapping.jl	85.43% <0.00%> (-0.42%)	⬇️
src/coord.jl	91.57% <0.00%> (-0.35%)	⬇️
src/geom/subplot.jl	95.36% <0.00%> (-0.24%)	⬇️
src/geom/segment.jl	94.44% <0.00%> (-0.16%)	⬇️
src/dataframes.jl	97.43% <0.00%> (-0.13%)	⬇️
... and 21 more

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[Mattriks]

I did the regression testing, and the main differences between master (with Compose 0.8.2) and dev (with Compose 0.9) are not due to this PR (rather GiovineItalia/Compose.jl#392). The main differences were in the tick labels for transformed axes with supercript exponents: now the tick labels are better spaced vertically (particularly for the x-axis).

Some plots have a trivial difference in line order due to this PR (i.e. whether a line is drawn above or below another), but nothing else about the plot is changed.