Infer shape parameter for hexbin from specified width and height?

[hafen]

The shape parameter in ly_hexbin is used to make sure hexagons have the right aspect ratio in the plot. This could be automatically inferred from the specified width and height if not specified.

Another good default for ly_hexbin would be to not allow resize (or it would be nice to have a resize tool option that preserves aspect ratio - like holding shift while resizing in photoshop).

Also, if the user specifies log=TRUE for an axis, that messes things up too.

Or perhaps it's best to just leave all of this to the user.