color_index and size_index should accept name of column in pandas DataFrame

[maxalbert]

Here is a slightly tweaked version of the Points example where the Points element is initialised from a pandas DataFrame instead of a numpy array.

import numpy as np
import pandas as pd

np.random.seed(10)
df = pd.DataFrame(np.random.rand(100, 4), columns=['col1', 'col2', 'col3', 'col4'])

Using an integer for color_index and size_index (as in the Elements tutorial) works as expected:

%%opts Points [color_index=2 size_index=3 scaling_factor=50]
hv.Points(df)

However, using the names of the data frame columns does not work (using latest master, e3a536d):

%%opts Points [color_index='col3' size_index='col4' scaling_factor=50]
hv.Points(df)

Traceback (most recent call last):
  File "/Users/albert/code/holoviews/holoviews/ipython/display_hooks.py", line 101, in wrapped
    max_branches = OutputMagic.options['max_branches'])
  File "/Users/albert/code/holoviews/holoviews/ipython/display_hooks.py", line 143, in element_display
    return renderer.html(element, fmt=renderer.fig)
  File "/Users/albert/code/holoviews/holoviews/plotting/renderer.py", line 211, in html
    plot, fmt =  self._validate(obj, fmt)
  File "/Users/albert/code/holoviews/holoviews/plotting/renderer.py", line 171, in _validate
    plot = self.get_plot(obj)
  File "/Users/albert/code/holoviews/holoviews/plotting/renderer.py", line 157, in get_plot
    plot = self_or_cls.plotting_class(obj)(obj, **plot_opts)
  File "/Users/albert/code/holoviews/holoviews/plotting/mpl/chart.py", line 23, in __init__
    super(ChartPlot, self).__init__(data, **params)
  File "/Users/albert/code/holoviews/holoviews/plotting/mpl/element.py", line 111, in __init__
    super(ElementPlot, self).__init__(element, **params)
  File "/Users/albert/code/holoviews/holoviews/plotting/plot.py", line 440, in __init__
    **dict(params, **plot_opts))
  File "/Users/albert/code/holoviews/holoviews/plotting/mpl/plot.py", line 88, in __init__
    super(MPLPlot, self).__init__(**params)
  File "/Users/albert/code/holoviews/holoviews/plotting/plot.py", line 179, in __init__
    super(DimensionedPlot, self).__init__(**params)
  File "/Users/albert/miniconda3/lib/python3.4/site-packages/param/parameterized.py", line 1021, in __init__
    self._setup_params(**params)
  File "/Users/albert/miniconda3/lib/python3.4/site-packages/param/parameterized.py", line 958, in override_initialization
    fn(parameterized_instance,*args,**kw)
  File "/Users/albert/miniconda3/lib/python3.4/site-packages/param/parameterized.py", line 1411, in _setup_params
    setattr(self,name,val)
  File "/Users/albert/miniconda3/lib/python3.4/site-packages/param/__init__.py", line 563, in __set__
    if not callable(val): self._check_value(val)
  File "/Users/albert/miniconda3/lib/python3.4/site-packages/param/__init__.py", line 692, in _check_value
    raise ValueError("Parameter '%s' must be an integer."%self._attrib_name)
ValueError: Parameter 'color_index' must be an integer.

[philippjfr]

Agreed, I've been meaning to allow this for a while now, should be pretty straightforward.

[philippjfr]

Implemented this in #461, should now work for all size_index and color_index plot options. Closing.