The docstring of ax.streamplot (“only”) states

x, y : 1d arrays
    an evenly spaced grid.

but looking at the actual code in the master branch (through the interface on GitHub website), it seems that a bit more of processing is involved (through the class matplotlib.streamplot.Grid) in order to also allow at least one special case of 2d-arrays for X and Y.

Expected behavior: such special casing being made explicite in the docstring.