Fix plotly colorbar positioning when multiple subplots are present

[stevenaleung]

Closes #2691

Contour plot example:

data1 = rand(3,3);
data2 = rand(3,3) .+ 10;
p1 = contour(data1, colorbar=:right)
p2 = contour(data2, colorbar=:right)
plot(p1, p2)

Left: before
Right: after

Heatmap example:

data1 = rand(3,3);
data2 = rand(3,3) .+ 10;
p1 = heatmap(data1, colorbar=:right)
p2 = heatmap(data2, colorbar=:right)
plot(p1, p2)

Left: before
Right: after

[codecov]

Codecov Report

Base: 90.95% // Head: 90.64% // Decreases project coverage by -0.30% ⚠️

Coverage data is based on head (70955d1) compared to base (68e20ae).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4582      +/-   ##
==========================================
- Coverage   90.95%   90.64%   -0.31%     
==========================================
  Files          40       41       +1     
  Lines        7802     8757     +955     
==========================================
+ Hits         7096     7938     +842     
- Misses        706      819     +113     

Impacted Files	Coverage Δ
src/init.jl	73.33% <0.00%> (-26.67%)	⬇️
src/backends.jl	81.50% <0.00%> (-13.35%)	⬇️
src/axes.jl	89.95% <0.00%> (-0.66%)	⬇️
src/backends/pgfplotsx.jl	88.88% <0.00%> (-0.13%)	⬇️
src/utils.jl	92.50% <0.00%> (-0.11%)	⬇️
src/consts.jl	100.00% <0.00%> (ø)
src/backends/gr.jl	91.65% <0.00%> (ø)
src/backends/pythonplot.jl	88.57% <0.00%> (ø)
src/plot.jl	97.77% <0.00%> (+0.01%)	⬆️
src/recipes.jl	84.58% <0.00%> (+0.03%)	⬆️
... and 6 more

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[BeastyBlacksmith]

wonderful, thank you!