There are actually a few things that are confusing me (based on the docs), and one thing that (given I understand correctly) could potentially be made better in the code. The first confusing thing in ...

This script is designed to test the number of walkers and number of steps needed for the emcee sampler to converge to a result. yerr = 0.1 * y_true + verr * np.random.rand(N) y = a_true * x**2 + ...