title: Graph-tool: Look up vertex by user defined labels
body: <p>I have been using networkx for all my network graphing application so far; however, this new project requires graphs with 10^8 nodes in the graph and networkx was not built to handle this number of nodes (it becomes incredibly slow), so I switched to <a href="https://graph-tool.skewed.de" rel="nofollow">graph-tool</a> which seems like it will be much faster. </p>
  
  <p>In using graph-tool I would like to label each vertex in the graph with a user defined systematic label; so that I may look them up by label and not by index. I have tried going over the <a href="https://graph-tool.skewed.de/static/doc/index.html" rel="nofollow">documentation</a>, but there doesn't seem to be any support for this; <a href="https://graph-tool.skewed.de/static/doc/graph_tool.html#graph_tool.PropertyMap" rel="nofollow">property maps</a> map from vertex_indices -> values, but I want the reverse. </p>
  
  <p>Is there actual support for this that I am missing? Otherwise is my best option just to create a python dictionary and map labels to vertex_indices that way? </p>