better error display for multidimensional populations

[drasmuss]

It would be very handy if we could get something like the "Plot distortion" display for multidimensional populations. It'd be pretty much impossible to visualize for >2 dimensions, but even an option to select 1 or 2 dimensions and have it project the plot into those two would be nice.

I'll work on this later, just putting this here to remind myself (or if anyone else is keen they can have a go).