color="r", lw=1,
label="True minimum")
plt.title("Convergence plot")
plt.xlabel("Number of calls $n$")
plt.ylabel(r"$\min f(x)$ after $n$ calls")
plt.grid()
After Change
if ax is None:
ax = plt.gca()
ax.set_title("Convergence plot")
ax.set_xlabel("Number of calls $n$")
ax.set_ylabel(r"$\min f(x)$ after $n$ calls")
ax.grid()
