Add contourplot

[t-bltg]

• add contourplot support (levels, colormap, colorbar controls)
• add Contour.jl dependency for computing levels using the Marching_squares algorithm
• add blend flag on all canvases to plot with/without color blend (avoid overlap issues on contour lines)
• add visible flag on all canvases: fix [FR] stand-alone version of printcolorbarrow to draw a colorbar in isolation #169 as a side effect
• add more cmaps: cividis, and some ANSI 256 subset colormaps.

https://github.com/JuliaGeometry/Contour.jl is a small requirement, since it only depends on StaticArrays.

See examples at JuliaPlots/Plots.jl#4031.

The new example from README.md now looks like this (gaussian profile):

tiny benchmark

using UnicodePlots, BenchmarkTools

main() = begin
  x = -3:.1:3
  y = -7:.1:3
  X = repeat(x', length(y), 1)
  Y = repeat(y, 1, length(x))
  g(x, y, x₀=0, y₀=-2, σx=1, σy=2) = exp(-((x - x₀) / 2σx)^2 - ((y - y₀) / 2σy)^2)
  z = map(g, X, Y)
  @btime heatmap($z)  #   15.090 ms (197665 allocations: 9.45 MiB)
  @btime contourplot($x, $y, $z)  #   2.833 ms (29227 allocations: 1.18 MiB)
end

main()

[codecov-commenter]

Codecov Report

Merging #199 (a49f96e) into master (65b9368) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
- Coverage   99.82%   99.74%   -0.09%     
==========================================
  Files          23       24       +1     
  Lines        1164     1179      +15     
==========================================
+ Hits         1162     1176      +14     
- Misses          2        3       +1     

Impacted Files	Coverage Δ
src/canvas/heatmapcanvas.jl	96.15% <ø> (-2.13%)	⬇️
src/colormaps.jl	100.00% <ø> (ø)
src/interface/barplot.jl	100.00% <ø> (ø)
src/canvas.jl	100.00% <100.00%> (ø)
src/canvas/asciicanvas.jl	100.00% <100.00%> (ø)
src/canvas/blockcanvas.jl	100.00% <100.00%> (ø)
src/canvas/braillecanvas.jl	98.07% <100.00%> (ø)
src/canvas/densitycanvas.jl	100.00% <100.00%> (ø)
src/canvas/dotcanvas.jl	100.00% <100.00%> (ø)
src/canvas/lookupcanvas.jl	100.00% <100.00%> (ø)
... and 8 more

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[johnnychen94]

It's almost impossible to review a 1000-lines PR with arbitrary unorganized commits and changes so I only left a few comments on changes I think should be reverted or separated. There are still many out there that I didn't comment.

It would be clearer to separate the visible and blend changes in separate PR(s), and then make this countourplot PR an incremental feature on top of those PRs.

FWIW, for a clean git history: it makes no sense to record how you evolve into the current status, instead, it makes sense to record how functionalities are built step by step. Thus if you add something in the first commit and then immediately delete it in the second commit, it should be purged from the git history.

[t-bltg]

Thanks for taking the time to reviewing this. The workflow is difficult since on this branch, I needed some colormap changes, and the integration / testing within Plots.jl is best if I apply the changes in a single branch, but I hear the concern on reviewing my PRs. Developing using multiple branches is also hard to follow, and wastes development time. Sorry about the code style changes, bad practice indeed. I will rebase the whole thing and split PRs.

[t-bltg]

Superseded by #204.