Support drawing origin for barplot length-scaling

[fedarko]

Continuation of #201. Currently, the length-scaling code doesn't take into account the sign of numeric values: all that's considered are the min and max values.

However, it would be nice to be able to have each bar point in different directions based on its sign -- see e.g. this sketch (definitely not to scale but the idea should be close enough)

This could be particularly useful for things like feature differentials, where we'd want +3 and -3 to be clearly symmetric.