Treat boolean color values as categorical

This issue is one of a number that relate to datashader support of timeseries.

```
>>> hv.__version__, bokeh.__version__, ds.__version__, panel.__version__, hvplot.__version__
('1.15.1', '2.4.3', '0.14.2', '0.13.1', '0.8.1')
```

```python
>>> timestamps = [pd.Timestamp('2022-04-01 0{0}:00:00-0000'.format(hour)) for hour in range(10)]
>>> df = pd.DataFrame({'timestamp':timestamps, 'SPY':np.random.rand(10), 'NDAQ':np.random.rand(10)})
>>> melted = pd.melt(df, id_vars=['timestamp'], var_name='stock', value_name='high')
>>> melted['bond_indicator'] = melted['stock'].isin(['SPY'])
```

Using `rasterize=True` gives a colorbar when a boolean value column can only be sensibly treated as categorical

There is another problem here when using `line_width=1` (or any non-zero) value which is a datashader issue (https://github.com/holoviz/datashader/issues/1133)