Correctly handle Gnuplot array/record semantics.
Correctly handle Gnuplot array/record semantics.

Gnuplot does not handle general binary data as a simple n-dimensional
array, but handles the last dimension specially, as if it were a struct.
Thus we need to treat shape[-1] specially in order to use data with more
than one `using' item.

This is an incompatible change.

marktsuchida/Xnuplot