Allow 2d coordinates for heatmap plots in PyPlot backend

[johnomotani]

Allows plotting on a non-rectangular grid, at least with PyPlot backend. For example:

using Plots
pyplot()

nx = 10
ny = 11

x = zeros(ny, nx)
y = zeros(ny, nx)

for i ∈ 1:nx, j ∈ 1:ny
    x[j,i] = i
    y[j,i] = 2.0 + j + i
end

z = randn(ny,nx)

heatmap(x,y,z)

savefig("test.png")

produces

Currently 'Draft' because:

• _heatmap_edges() is fudged for 2d x and y. PyPlot can handle the cell-centre/cell-edge conversion automatically so the _heatmap_edges() functionality is not needed for the case I was trying to get working. It would be nicer to be more general, but the logic is a bit more complicated (I guess an extra argument would need to be passed to say which dimension needs to be converted from centres to edges?) - is it worth doing and does anyone have opinions/suggestions on the best way?
• I guess a test would be good? I would be very grateful for a pointer on where the test should go and if there is something similar I could copy to start from.
• What to do about other backends? I tried running the same test case with GR and got an error that seemed to happen due to z being transposed. I got lost trying to look for where a special-case for 2d x and y would be needed though... Should only backends where 2d x and y are known to work be supported? If so where would be a good place to throw an error for unsupported backends?

Maybe an expert can see a better way to implement this and would find it easier to start from scratch, in which case please consider this just a feature request!

[codecov]

Codecov Report

Merging #4298 (3133ee8) into master (6d537c6) will decrease coverage by 0.18%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master    #4298      +/-   ##
==========================================
- Coverage   79.92%   79.74%   -0.19%     
==========================================
  Files          29       29              
  Lines        6969     6986      +17     
==========================================
+ Hits         5570     5571       +1     
- Misses       1399     1415      +16     

Impacted Files	Coverage Δ
src/utils.jl	72.87% <0.00%> (-1.80%)	⬇️
src/axes.jl	86.27% <66.66%> (-0.16%)	⬇️

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