WIP: Polar coordinates

[simleb]

I'm trying to implement polar coordinates. I followed @dcjones's suggestion in #610 (although it was for ternary coordinates) to add a new Coord and to make geometries and guides dispatch on the coordinate type. So far this is just a proof of concept, help would be more than welcome!

I introduced two new aesthetics: rho (aliases: r and ρ) and phi (alias ϕ), so one doesn't have to specify Coord.polar explicitly. I followed ISO 31-11 for the names.

I chose to dispatch element_coordinate_type on the mapped aesthetics as well as the geometry. Not sure if this is the best way.

The biggest issue is with Guides. I don't understand the scale at which they are drawn. Right now the circular grid is too small while the angular grid doesn't even show up.

Eventually, better Stats need to be implemented to compute the grid correctly but I would like to have something basic working.

Here is what I have so far:

plot(ρ=linspace(0, 5, 50), ϕ=linspace(0, 5, 50), Geom.point)

[tlnagy]

Hey @simleb, are you still working on this? This would be a great thing to have in Gadfly.

[simleb]

No, I'm afraid I won't have time to work on it for a while. Feel free to take over.