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

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

        return self.x_coord.get_tics(self.x_range[0], self.x_range[1], interval)