Bug in NumberPlane when using x_max (and likely the y/min values too)

[XorUnison]

NPac=NumberPlane(x_max=20).add_coordinates()
class Test(Scene):
    def construct(self):
        self.add(NPac.shift([-15,0,0]))
        self.wait(1)

This results in the horizontal lines not being anywhere near as long as they should be. Leaving this here for either someone else to investigate or fix, or in case of doubt myself coming back to this some point in the not too close future.