matplotlib - fill open path inside U.S. borders

I am trying to plot filled paths over a U.S. map. I plot the given paths, but since these are not closed paths, the filling is truncated at a line drawn between last and first point in the path. I want to extend the path to the United States border instead.

I am no expert in matplotlib or GIS. Could anyone help me find documentation or blogs discussing this type of issue? I am convinced somebody else has already faced this problem although I have not been able to find any paper, documentation, blog on it.

Thank you