1D plots with coord on vertical axis

If I have a 1D cube with a coordinate representing something in the vertical, say depth in the ocean, then I can't plot it as a profile using `iris.plot.plot` as this will always plot the depth coordinate along the x-axis.

There should be a mechanism for telling `iris.plot.plot` that you want to do this and/or some cleverness to automatically detect vertical coordinates and transpose 1D plots.
