I wasn't sure how to word that best, basically : http://paste.fedoraproject.org/178311/19358142 If the width request is changed to something more usual, the line is correctly plotted.
so that in order for plot() to maintain its current capabilities, along with a more limited set of capabilities for the ax=Axes case, the code would become needlessly complex, branching this way and that depending on whether the caller passed in an Axes instance or not. ...