Update _chart_types.py

[yoonghm]

barnorm is default to None instead of ''

[yoonghm]

The default parameter barnorm in bar_polar() should be None, as stated in the document rather than the code. Please review.

[jonmmease]

Hi @yoonghm , thanks for your interest. You're right that plotly.py does not explicitly include the default values of every property, and this is by design. When a property is None on the Python side, this means that that the default will be chosen by the JavaScript library.

Is there a particular problem that you were trying to solve?

[nicolaskruchten]

This seems like a good modification, I can't think of a valid reason for it to be set to "" in bar_polar() and None in histogram().

[nicolaskruchten]

We should think about adding barnorm to bar() :)