reduce memory footprint and plotting efficiancy: default to f32 plotting

I am quite surprised that PlotPoint and other data is stored as f64 inside the plot state. The majority of plotting and rendering backends default to single precision f32, especially when rendering with gpu backends. Nevertheless i am quite curious where the decision came from to go for f64 here instead?

Since it is beneficial for memory-consumption and performance, especially when rendering alot of primitives i would suggest that the default precision is more reasonable for a single precision setup.

also related to:
#18