static void GraphFunc (Gtk.TreeViewColumn tree_column, Gtk.CellRenderer cell, Gtk.TreeModel model, Gtk.TreeIter iter)
{
var renderer = (RevisionGraphCellRenderer)cell;
Gtk.TreeIter node;
model.GetIterFirst (out node);
renderer.FirstNode = node.Equals (iter);
model.IterNthChild (out node, model.IterNChildren () - 1);
renderer.LastNode = node.Equals (iter);
}