wtk_graph.asy: Fix palette positioning in graphMatrixFile

wtk_graph.asy: Fix palette positioning in graphMatrixFile

We know where the figure axes end (k_u0 and dx) in graph coordinates,
and the palette() function expects and initial and final pair in graph
coordinates, but we want the palette offset to be in frame
coordinates.  This means we need to take our figure corners from graph
coordinate through picture coordinates to frame frame coordinates, add
our offset, and go back through picture coordinates to graph
coordinates before passing the values in to palette().