[graph-tool] Error in drawing graphs

Hi, I am trying the example code for drawing the graphs on my machine. Here is the error I am getting. I am sure this is something to do with the installed packages. I am using Debian and am not sure what package or path is missing. ****** Traceback (most recent call last): File "gt_draw_graph.py", line 23, in <module> penwidth=ebet, overlap="prism", output="graph-draw.png") File "/usr/local/lib/python2.5/site-packages/graph_tool/draw/__init__.py", line 264, in graph_draw gvg = gv.graph("G") NameError: global name 'gv' is not defined ****** TIA, Karthik