I want to plot trees using Python. Decision trees, Organizational charts, etc. Any library that helps me with that?
There's graphviz - http://www.graphviz.org/. It uses the "DOT" language to plot graphs. You can either generate the DOT code yourself, or use pydot - https://github.com/pydot/pydot. You could also use networkx - http://networkx.lanl.gov/tutorial/tutorial.html#drawing-graphs, which make it easy to draw to either graphviz or matplotlib.
networkx + matplotlib + graphviz gives you the most flexibility and power, but you need to install a lot.
If you want a quick solution, try:
Install Graphviz.
open('hello.dot','w').write("digraph G {Hello->World}")
import subprocess
subprocess.call(["path/to/dot.exe","-Tpng","hello.dot","-o","graph1.png"])
# I think this is right - try it form the command line to debug
Then you install pydot, because pydot already does this for you. Then you can use networkx to "drive" pydot.