title: Graph Tool's edge_gradient property
body: <p>I would like to use the <code>edge_gradient</code> property on Graph Tool's <code>gt.graph_draw()</code> in order to better visualize the direction of connections in plots which are too crowded for markers such as arrows.</p>
  
  <p>From the description in the docs, it seems this is what this property should do. Currently, however, it only lets me set the edges to a solid color.</p>
  
  <p>I am using the property like so:</p>
  
  <pre><code>egradient = g.new_edge_property('vector&lt;double&gt;')
  g.edge_properties['egradient'] = egradient
  e = g.add_edge(v1, v2)
  egradient[e] = (0.9, 0.329,0.282,0.478,1)
  ...
  gt.graph_draw(g, ... edge_gradient=g.edge_properties["egradient"])
  </code></pre>
  
  <p>The appearance remains unchanged if I modify the first value in <code>(0.9, 0.329,0.282,0.478,1)</code> - and if I try to pass it a list of tuples I get this from the graph tool internals:</p>
  
  <p><code>
  TypeError: float() argument must be a string or a number
  </code></p>
  
  <p>How can I achieve what I am looking for in graph tool? If I can't, then what else is the first value in the edge gradient 5-tuple actually good for?</p>