Repeated subplots in plot.py file

[kevinqyh0827]

Hi, there,

When I was running the example of RRT* star methods, the visualization result has the problem that multiple axes exist at the same time as shown below.

To fix that, a possible solution is:

1. Locate the plotEnv() function at /utlils/plot/plot.py
2. Comment line 54 since it creates another subplot.
3. Change the "ax" from line 57, 66, 75 to "self.ax" to reuse the already existed subplots.