v1.4.1

• specify the license (BSD 3-clause)
• prevent creation of duplicate nodes or groups
• allow the creation of edges between groups
• rename edge_color/edge_type/edge_width to border_color/border_type/border_width for nodes and groups
• allow creation of labels at either end of edge, by providing optional source_label and target_label arguments to add_edge()
• allow control of edge label colors using the label_background_color and label_border_color options to add_edge()