[Chart] Bezier smoothing

[Sacha Sokoloski]

I'm just raising this point again to see if anyone feels inspired to 
answer it.

So I'm still sitting on some nice code for generating contour plots in 
Chart which I think would be of interest, but the primary problem is 
that to make contour plots look good you need some kind of spline 
interpolation. In general having the ability to apply spline 
interpolation in Chart might be nice. I notice that Diagrams provides 
tools for this, but of course we want to retain the ability to render in 
the cairo backend directly, which means we would need some kind of 
'native' tool.

Anyone have any thoughts about this?