Low resolution when applying nonlinear transformation

I want to apply a **nonlinear** transformation to a number plane but...it won't give a correct result.(No problem with linear transformation)
Expected:
What I've got instead:
,then
and, finally,

------

I'm using the latest version of manim with **Python 3.7**, and here is my code.
```python
#!/usr/bin/env python
from big_ol_pile_of_manim_imports import *

class Scene1(Scene):
def construct(self):
pl = NumberPlane()
x_axis = NumberLine()
y_axis = NumberLine()
y_axis.rotate(np.pi / 2)
rt = Rectangle(height=4, width=8)
rt.set_fill(GREEN, opacity=0.5)
self.play(ShowCreation(pl), ShowCreation(x_axis), ShowCreation(y_axis))
self.play(FadeIn(rt))

self.play(
*[ApplyPointwiseFunction(lambda point: complex_to_R3(R3_to_complex(point) ** 2),mob) for mob in self.mobjects]
)
self.wait()
```

As you can see in the last picture, the transformation has been **correctly** applied to **the rectangle and the axies**(`NumberLine`), but those grid lines tend to be still straight, I mean not smooth.

It seems that there's no problem with the transformation function itself, right (I guess)?

What the author 3b1b used in that video may be just grid lines (`Grid`), and it can produce the correct result. But how can I apply a pointwise function to the whole `NumberPlane`, or, something like `ComplexPlane` ?
Thanks.