Modified plot_improvement.py to fix agent directory sorting bug

plot_improvement.py

@@ -19,7 +19,9 @@ def calc_stats(filepath):
 # args = parser.parse_args()
 
 filepaths = []
-for d in os.listdir("agents/"):
+agent_dirs = os.listdir("agents/")
+agent_dirs.sort()
+for d in agent_dirs:
     if "dqn_v2" in d:
         path = "agents/" + d + "/evaluations.npz"
         filepaths.append(path)