Re-added the red line

src/1_einstein_model.md

@@ -261,7 +261,7 @@ xline = [1, 1];
 yline = [0, 1.5];
 fig, (ax, ax2) = pyplot.subplots(ncols=2, figsize=(10, 5))
 omega = np.linspace(0.1, 2)
-ax.plot(omega, 1/(np.exp(omega) - 1), '-')
+ax.plot(omega, 1/(np.exp(omega) - 1), '-', xline, yline, 'r--')
 ax.set_ylim(0, top=3)
 ax.set_xlim(left=0)
 ax.set_xlabel('$\hbar \omega$')