Add `subcoordinate_y` keyword

[maximlt]

Closes https://github.com/holoviz/hvplot/issues/1160

• [x] Add tests

import hvplot.pandas  # noqa
from bokeh.sampledata.sea_surface_temperature import sea_surface_temperature as sst

new_cols = {f'temperature{i}': sst.temperature.shift(i*1000) for i in range(1, 4)}
sst.assign(**new_cols).hvplot(subcoordinate_y=True, legend=False)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.89%. Comparing base (feffee5) to head (6316666).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1379 +/- ## ========================================== + Coverage 88.87% 88.89% +0.02% ========================================== Files 51 51 Lines 7478 7494 +16 ========================================== + Hits 6646 6662 +16 Misses 832 832 ```

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[droumis]

Looks pretty good!

Code

```python import numpy as np import pandas as pd from pathlib import Path import mne import colorcet as cc from hvplot import pandas hv.extension('bokeh') # data from https://datasets.holoviz.org/eeg/v1/S001R04.edf data_path = raw = mne.io.read_raw_edf(data_path, preload=True) raw.set_eeg_reference("average") raw.rename_channels(lambda s: s.strip(".")); df = raw.to_data_frame() df.set_index('time', inplace=True) df.hvplot(subcoordinate_y={'subcoordinate_scale':4}, responsive=True, min_height=700, legend=False, color='black', line_width=1) ```

Known issues:

• https://github.com/holoviz/holoviews/issues/6252

Nice to haves eventually:

• apply a color key/cmap based on curve label. for instance, color grouping based first letter of columns:

  entries = df.columns.tolist()
  colors = cc.b_glasbey_bw
  letter_to_color = {letter: colors[i % len(colors)] for i, letter in enumerate(sorted(set(e[0] for e in entries)))}
  entry_to_color = {entry: letter_to_color[entry[0]] for entry in entries}
  df.hvplot(subcoordinate_y={'subcoordinate_scale':4}, responsive=True, min_height=700, legend=False, line_width=1, cmap=entry_to_color)

• range linked minimap