code: def plot_sigma_convergence(model: SklearnModel, ax=None):
      if ax is None:
          fig, ax = plt.subplots(1, 1)
      sigma_samples = [
          x.sigma.current_value() for x in model.model_samples
      ]
      ax.plot(sigma_samples)
      ax.set_title("Sigma Convergence")
      ax.set_xlabel("Iteration")
      ax.set_ylabel("Sigma")
      return ax
  
label_desc: Use image node_id 1 for calling a global function with example usage: plot_sigma_convergence(model, ax) and returns: ax