Generate a GraphViz Dot graph.

        Returns a GraphViz Digraph object.

def build_dot(self):
        """Generate a GraphViz Dot graph.

        Returns a GraphViz Digraph object.
        """
        from graphviz import Digraph

        # Build GraphViz Digraph
        dot = Digraph()
        dot.attr("graph", 
                 bgcolor=self.theme["background_color"],
                 color=self.theme["outline_color"],
                 fontsize=self.theme["font_size"],
                 fontcolor=self.theme["font_color"],
                 fontname=self.theme["font_name"],
                 margin=self.theme["margin"],
                 rankdir="LR",
                 pad=self.theme["padding"])
        dot.attr("node", shape="box", 
                 style="filled", margin="0,0",
                 fillcolor=self.theme["fill_color"],
                 color=self.theme["outline_color"],
                 fontsize=self.theme["font_size"],
                 fontcolor=self.theme["font_color"],
                 fontname=self.theme["font_name"])
        dot.attr("edge", style="solid", 
                 color=self.theme["outline_color"],
                 fontsize=self.theme["font_size"],
                 fontcolor=self.theme["font_color"],
                 fontname=self.theme["font_name"])

        for k, n in self.nodes.items():
            label = "<tr><td cellpadding='6'>{}</td></tr>".format(n.title)
            if n.caption:
                label += "<tr><td>{}</td></tr>".format(n.caption)
            if n.repeat > 1:
                label += "<tr><td align='right' cellpadding='2'>x{}</td></tr>".format(n.repeat)
            label = "<<table border='0' cellborder='0' cellpadding='0'>" + label + "</table>>"
            dot.node(str(k), label)
        for a, b, label in self.edges:
            if isinstance(label, (list, tuple)):
                label = "x".join([str(l or "?") for l in label])

            dot.edge(str(a), str(b), label)
        return dot