Ensure .stack works on Areas with different vdims

[TheoMathurin]

Fixes #5689

I have yet to try this on main, but works fine on 1.15.4.

[codecov-commenter]

Codecov Report

Merging #5693 (02ac3a2) into main (5ec8e63) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5693   +/-   ##
=======================================
  Coverage   88.30%   88.30%           
=======================================
  Files         302      302           
  Lines       62605    62630   +25     
=======================================
+ Hits        55281    55305   +24     
- Misses       7324     7325    +1     

Impacted Files	Coverage Δ
holoviews/element/chart.py	96.84% <100.00%> (ø)
holoviews/tests/plotting/bokeh/test_areaplot.py	100.00% <100.00%> (ø)
...loviews/tests/plotting/matplotlib/test_areaplot.py	100.00% <100.00%> (ø)
holoviews/tests/plotting/plotly/test_areaplot.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[hoxbro]

Thank you. The changes look good to me. Will it be possible for you to add a unit test? You can use the original example and verify the Baseline is correct. This can be found here:

The test can be added here:
https://github.com/holoviz/holoviews/blob/main/holoviews/tests/plotting/bokeh/test_areaplot.py

[TheoMathurin]

Ok, maybe something along these lines?

df = pd.DataFrame({'x': [1, 2, 3], 'y_1': [1, 2, 3], 'y_2': [6, 4, 2], 'y_3': [8, 1, 2]})
overlay = hv.Overlay([hv.Area(df, kdims='x', vdims=col, label=col) for col in ['y_1', 'y_2', 'y_3']])

plot = hv.Area.stack(overlay)
baselines = [np.array([0, 0, 0]), np.array([1., 2., 3.]), np.array([7., 6., 5.])]

for n, baseline in zip(plot.data, baselines):
    self.assertEqual(plot.data[n].data.Baseline, baseline)

[hoxbro]

Yes. That looks good to me.

[TheoMathurin]

Alright I guess the test should be included for all three backends? I see that there are test_areaplot modules for plotly and matplotlib as well.

There's also the test_operation module which already contains tests on the stack operation. I don't know if it could be an alternative.

[hoxbro]

Alright I guess the test should be included for all three backends?

It would be nice, but it is up to you if you want to do it.

There's also the test_operation

Just ignore this. Operations are pretty cumbersome, and we want to add a test for your simple example.

[hoxbro]

Thank you for PR 👍

[TheoMathurin]

You're welcome!