Ensure date sorting of multi index Pandas dataframes

[maximlt]

Fixes #1361

Also:

• Cleans up some imports in the test suite, since pandas and panel are direct dependencies of hvPlot their import doesn't need to be guarded
• Update some test code to HoloViews 1.19

[codecov]

Codecov Report

Attention: Patch coverage is 98.03922% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.54%. Comparing base (6c96c7e) to head (7b978e2).
Report is 6 commits behind head on main.

Files	Patch %	Lines
hvplot/tests/testutil.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1363      +/-   ##
==========================================
+ Coverage   87.39%   88.54%   +1.15%     
==========================================
  Files          50       50              
  Lines        7490     7362     -128     
==========================================
- Hits         6546     6519      -27     
+ Misses        944      843     -101     

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[ahuang11]

Looks good to me

import hvplot.pandas
import pandas as pd
import numpy as np


# datetime column with groups and value

df = pd.DataFrame({
    'datetime': pd.date_range('2020-01-01', periods=100, freq='D'),
    'group': ['A', 'B', 'C', 'D'] * 25,
    'value': np.random.randn(100)
})
df = df.sample(frac=1).set_index(["datetime", "group"])

df.hvplot(x='datetime', y='value', by='group', kind='line')