Return the y position (on the canvas) corresponding to YVAL.

def y_pos(self, yval):
        "Return the y position (on the canvas) corresponding to YVAL."
        off = self.y_coord.get_canvas_pos(self.size[1], yval,
                                          self.y_range[0], self.y_range[1])
        return self.loc[1] + off