Return the list of Y values for which tick marks and grid lines are drawn.

def y_tic_points(self, interval):
        "Return the list of Y values for which tick marks and grid lines are drawn."
        if type(interval) == FunctionType:
            return interval(*self.y_range)

        return self.y_coord.get_tics(self.y_range[0], self.y_range[1], interval)