def write_frame_png(self, filename="nix-tree.png"):
    """Dump the graph to a png file"""

    try:
        cmap = getattr(matplotlib.cm, self.config["color_map"])
    except AttributeError:
        raise util.TreeCLIError("Colormap {} does not exist".format(
            self.config["color_map"]))

    pos = {n: (n.x, n.y) for n in self.nodes}
    col_scale = 255.0/(self.depth+1.0)
    col = [(x.level+random.random()*self.config["color_scatter"])*col_scale
           for x in self.G.nodes()]
    col = [min([x,255]) for x in col]

    img_y_height=self.config["img_y_height_inches"]

    size_min = self.config["min_node_size"]
    size_max = self.config["max_node_size_over_min_node_size"] * size_min

    plt.figure(1, figsize=(img_y_height*self.config["aspect_ratio"],
                           img_y_height))
    node_size = [min(size_min + (x.out_degree-1)*
                     self.config["add_size_per_out_link"],
                     size_max) if x.level > 0 else size_max for
                 x in self.G.nodes()]

    # Draw edges
    nx.draw(self.G, pos, node_size=node_size,  arrows=False,
         with_labels=self.config["show_labels"],
         edge_color=self.config["edge_color"],
         font_size=12*self.config["font_scale"],
         node_color=col, vmin=0, vmax=256,
         width=self.config["edge_width_scale"],
         alpha=self.config["edge_alpha"], nodelist=[])

    # Draw nodes
    nx.draw(self.G, pos, node_size=node_size,  arrows=False,
         with_labels=self.config["show_labels"],
         font_size=12*self.config["font_scale"],
         node_color=col, vmin=0, vmax=255, edgelist=[],
         font_weight="light", cmap=cmap,
         font_color=self.config["font_color"])

    logger.info("Writing png file: {}".format(filename))
    plt.savefig(filename, dpi=self.config["dpi"])
    plt.close()