also plot center of mass

plots/animation.py

@@ -20,6 +20,7 @@ def animate(i):
     yi = y[i*N:(i+1)*N]
     plt.cla()
     plt.scatter(xi, yi, s=0.1)
+    plt.scatter(np.sum(xi)/N, np.sum(yi)/N, s=1, c="red")
     plt.xlabel("x")
     plt.ylabel("y")
     plt.axis("equal")