Using 'groups' in call to graph_tool.draw.sfdp_layout()

Greetings,

I have been unable to see any effect on the layout when using the 'groups'
argument.

For example, I have tried setting groups equal to a VertexPropertyMap as
follows:

        vprop_layout_groups = self.new_vertex_property("int") # Vertex
groups.

With layout_groups = [0 0 0 0 0 0 1 1 1]
I can see no grouping effect on the positions of the vertices as returned by
sfdp_layout().
The three vertices without edges are in the '1' group. The connected
vertices are in the '0' group.
<https://nabble.skewed.de/file/t496263/sfdp-groups.png&gt;
I have been unable to find any example showing a call to sfdp_layout() where
'groups' is used.
Is there a working example available?

Thank you.

p.s. Is there a way to donate money to the graph-tool project?

You should experiment with the `gamma` parameter which controls the
strength of the attractive force between nodes of the same group.