Return the x position (on the canvas) corresponding to XVAL.

def x_pos(self, xval):
        "Return the x position (on the canvas) corresponding to XVAL."
        off = self.x_coord.get_canvas_pos(self.size[0], xval,
                                          self.x_range[0], self.x_range[1])
        return self.loc[0] + off