Visualizing cell growth I model the following events: a rod-shaped cell with center $x$ grows symmetrically until it reaches a maximum length/age, it divides into two identical cells, and the process continues. At division, the direction angle $\varphi$ of the daughter cells changes randomly. 

So, the growth rate is $l=\frac{l_{max}}{2}+\frac{a}{a_{max}}\frac{l_{max}}{2}$

Is there any way of naively visualizing this? I found different cell modellers on GitHub but they were very complicated biologically speaking. I have some basic Python knowledge, but I have never done simulations.
I would recommend looking into Python's plotting utilities, for example here.

I would recommend specifically looking at the examples that plot mathematical functions.

The basic idea is that you create a function that computes your equation based on the different parameters as inputs ($a, a_{max}, l_{max}$) and then you can plug in a range of values for each parameter and plot e.g. with $a_{max}$ on the x axis, then different lines showing a range of values for $l_{max}$, or whatever makes sense for you.

e.g. 

import numpy as np
import matplotlib.pyplot as plt

def fn(a, amax, lmax):
  return lmax/2 + (a/amax) * (lmax/2)

plt.figure()
amax = np.arange(0.01, 1, .01)
plt.plot(amax, fn(.005, amax, .1), "ro") ## red points
plt.plot(amax, fn(.005, amax, .2), "go") ## green point
plt.show()


I have no idea if these parameter ranges are meaningful for you, you will have to figure that out for yourself.