backend_gtk and expose_event

I guess this can be considered a bug, or not?

Yes, it's a bug. We try to handle invalid transformations w/o
failing.

I think the easiest solution might be to use nans in the transform
module rather than raise an exception when transforming an array. And
the clients of the transform, eg the legend auto-scaling code, can
ignore the nans when deciding where to place the legend.

JDH