Check cmap type before comparing to categories

[Azaya89]

fixes #1461 where the use of matplotlib colormap objects as cmap argument results in "TypeError: argument of type 'LinearSegmentedColormap' is not iterable...."

import hvplot.xarray  # noqa
from matplotlib import colormaps
import xarray as xr

ds = xr.tutorial.open_dataset("air_temperature")
da = ds.air.isel(time=0)

da.hvplot.image(cmap=colormaps['viridis'])

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.96%. Comparing base (9078378) to head (1393fd1).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
hvplot/util.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1462      +/-   ##
==========================================
+ Coverage   88.94%   88.96%   +0.01%     
==========================================
  Files          52       52              
  Lines        7781     7812      +31     
==========================================
+ Hits         6921     6950      +29     
- Misses        860      862       +2     

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[ahuang11]

Thanks! I think it fixes the issue.

One thing you could test is whether a categorical cmap as a string and LinearSegmentedColormap actually gets processed as expected.

[Azaya89]

One thing you could test is whether a categorical cmap as a string and LinearSegmentedColormap actually gets processed as expected.

Maybe I could do that in another PR since this one has solved the original issue.

[maximlt]

LGTM!

One thing you could test is whether a categorical cmap as a string and LinearSegmentedColormap actually gets processed as expected.

@Azaya89 has been working on cataloguing the different ways usage and values accepted by cmap/color_key/color/c/etc. Once we have a good reference document, we will be able to more easily identify gaps in the test suite and fix them. Let's do this in another PR to get the fix of this one merged and released.