[WIP] Add value_labels option to bar plots

[jsignell]

Closes #375

Example:

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

# create some sample data
df = pd.DataFrame({
    'col1': ['bar1', 'bar2', 'bar3'] * 2,
    'col2': np.random.rand(6),
    'col3': ['apple',] * 3 + ['orange'] * 3
})

# plot
df[df.col3 == 'apple'].hvplot(kind='bar', x='col1', y='col2', ylim=(0, 1.2), value_labels=True)

NOTE: This doesn't work for cases where the data are grouped along more than one dimension on the x axis. AFAICT labeling those isn't possible in holoviews. For instance:

df.hvplot(kind='bar', x='col1', y='col2', ylim=(0, 1.2), by="col3")

df.hvplot(kind='bar', x='col1', y='col2', ylim=(0, 1.2), by="col3", value_labels=True)

------------------------------------------------------------------
ValueError                       Traceback (most recent call last)
<ipython-input-24-b9ea1072518c> in <module>
----> 1 df.hvplot(kind='bar', x='col1', y='col2', ylim=(0, 1.2), by="col3", value_labels=True)

~/holoviz/hvplot/hvplot/plotting/core.py in __call__(self, x, y, kind, **kwds)
     70                 return pn.panel(plot, **panel_dict)
     71 
---> 72         return self._get_converter(x, y, kind, **kwds)(kind, x, y)
     73 
     74     def _get_converter(self, x=None, y=None, kind=None, **kwds):

~/holoviz/hvplot/hvplot/converter.py in __call__(self, kind, x, y)
   1023                     dataset = Dataset(data)
   1024                     dataset = dataset.redim(**self._redim)
-> 1025                 obj = method(x, y)
   1026                 obj._dataset = dataset
   1027 

~/holoviz/hvplot/hvplot/converter.py in bar(self, x, y, data)
   1388             if self.value_labels:
   1389                 labels = Labels(obj.data, obj.kdims + obj.vdims, obj.vdims).opts(text_baseline="bottom")
-> 1390                 return obj * labels
   1391             return obj
   1392         return self._category_plot(Bars, x, list(y), data)

~/conda/envs/dask-dev/lib/python3.7/site-packages/holoviews/core/data/__init__.py in __init__(self, data, kdims, vdims, **kwargs)
    351                                            datatype=kwargs.get('datatype'))
    352         (data, self.interface, dims, extra_kws) = initialized
--> 353         super(Dataset, self).__init__(data, **dict(kwargs, **dict(dims, **extra_kws)))
    354         self.interface.validate(self, validate_vdims)
    355 

~/conda/envs/dask-dev/lib/python3.7/site-packages/holoviews/core/dimension.py in __init__(self, data, kdims, vdims, **params)
    847             params['cdims'] = {d if isinstance(d, Dimension) else Dimension(d): val
    848                                for d, val in params['cdims'].items()}
--> 849         super(Dimensioned, self).__init__(data, **params)
    850         self.ndims = len(self.kdims)
    851         cdims = [(d.name, val) for d, val in self.cdims.items()]

~/conda/envs/dask-dev/lib/python3.7/site-packages/holoviews/core/dimension.py in __init__(self, data, id, plot_id, **params)
    506             params['group'] = long_name
    507 
--> 508         super(LabelledData, self).__init__(**params)
    509         if not util.group_sanitizer.allowable(self.group):
    510             raise ValueError("Supplied group %r contains invalid characters." %

~/conda/envs/dask-dev/lib/python3.7/site-packages/param/parameterized.py in __init__(self, **params)
   2338 
   2339         self.param._generate_name()
-> 2340         self.param._setup_params(**params)
   2341         object_count += 1
   2342 

~/conda/envs/dask-dev/lib/python3.7/site-packages/param/parameterized.py in override_initialization(self_, *args, **kw)
    989         original_initialized=parameterized_instance.initialized
    990         parameterized_instance.initialized=False
--> 991         fn(parameterized_instance,*args,**kw)
    992         parameterized_instance.initialized=original_initialized
    993     return override_initialization

~/conda/envs/dask-dev/lib/python3.7/site-packages/param/parameterized.py in _setup_params(self_, **params)
   1200                 self.param.warning("Setting non-parameter attribute %s=%s using a mechanism intended only for parameters",name,val)
   1201             # i.e. if not desc it's setting an attribute in __dict__, not a Parameter
-> 1202             setattr(self,name,val)
   1203 
   1204     @classmethod

~/conda/envs/dask-dev/lib/python3.7/site-packages/param/parameterized.py in _f(self, obj, val)
    294             instance_param.__set__(obj, val)
    295             return
--> 296         return f(self, obj, val)
    297     return _f
    298 

~/conda/envs/dask-dev/lib/python3.7/site-packages/param/parameterized.py in __set__(self, obj, val)
    821             val = self.set_hook(obj,val)
    822 
--> 823         self._validate(val)
    824 
    825         _old = NotImplemented

~/conda/envs/dask-dev/lib/python3.7/site-packages/param/__init__.py in _validate(self, val)
   1381             if min_length is not None and max_length is not None:
   1382                 if not (min_length <= l <= max_length):
-> 1383                     raise ValueError("%s: list length must be between %s and %s (inclusive)"%(self.name,min_length,max_length))
   1384             elif min_length is not None:
   1385                 if not min_length <= l:

ValueError: kdims: list length must be between 2 and 2 (inclusive)

[jsignell]

I'm happy to fix this up if it is something that hvplot would like to support.

[jbednar]

I'd love to see that supported!