Geom.blank

[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

• Adds Geom.blank
• Adds plot() (closes Zero-argument function definition missing for plot() #947) which is user-friendly, and allows push!
• Adds docstring for push!, plus doc examples
• Is not aimed at the case with explicit empty arrays: x=[], y=[] (creating empty plot #751), which should be handled separately

Example

p1 = plot(x=[0,10], y=[0,1], Geom.blank)
p2 = plot()
p3 = push!(plot(), layer(x=1:5, y=rand(5), Geom.point))
hstack(p1, p2, p3)

[bjarthur]

docs are timing out, despite the recent PR adding travis_wait.

[Mattriks]

This PR is running travis with the old travis.yml (before #1343). I'll wait til you merge #1347 and #1348, then rebase.

[codecov-io]

Codecov Report

Merging #1345 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1345      +/-   ##
==========================================
- Coverage   90.15%   90.03%   -0.12%     
==========================================
  Files          36       37       +1     
  Lines        3940     3944       +4     
==========================================
- Hits         3552     3551       -1     
- Misses        388      393       +5

Impacted Files	Coverage Δ
src/geometry.jl	75% <ø> (ø)	⬆️
src/Gadfly.jl	77.38% <0%> (-0.2%)	⬇️
src/geom/blank.jl	0% <0%> (ø)
src/aesthetics.jl	81.03% <0%> (-0.87%)	⬇️

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[Mattriks]

Done.