Add Polar visualization.

[JFerrerBeired]

Create a new plot type (child of ScatterChartBase) that allows for polar representation:

It will take a call to plot_funciton like the other ScatterChartBase childs, but instead of using x and y it will use phi and r representing the angle and radius (distance from the center).

I still have to think, however, on the particular implementation, since some things will have to be completely changed (like the tics positions), but most of the other functions should be reusable in the current cartesian form.

[fenix-hub]

Why should this be a child of ScatterChartBase ? Inheriting from Chart should give you right the base you need to do less changes in the future. Also if you might need it take a look at RadarChart which is already implemented and very similar.