Hotfixes/charts

Issues: fixes #1475  fixes #1490 

Note: except for subitems:

* decide about current _charts.Chart.plot implementation (based on property to support facet)
* choose a better way to deal with the high number of charts keywords arguments

that are going to be managed on separate self contained PRs

So, fixes:

* collect convenience methods like donut.polar_to_cartesian somewhere meaningful ad not in specific chart modules
* inputs conversion to DataAdapter on charts should be grouped on a common place (on get_data?) to improve conceptual understanding
* redefine xgrid and ygrid as instance attrs and not class
Facet... I've changed to explicitly specify facet. Much easier to understand.