code: def graph(self, **kw):
          
          self.name = kw.get('name', 'G')
          self.fillcolor = kw.get('fillcolor', '#ffffff')
          self.fontcolor = kw.get('fontcolor', '#000000')
          self.rankdir = kw.get('rankdir', 'BT' if self.reverse else 'TB')
          if kw.get('concentrate', True):
              self.concentrate = 'concentrate = true;'
          else:
              self.concentrate = ''
          self.dedent("""
              digraph {self.name} {{
                  {self.concentrate}
                  rankdir = {self.rankdir};
                  node [style=filled,fillcolor="{self.fillcolor}",fontcolor="{self.fontcolor}",fontname=Helvetica,fontsize=10];
  
          """.format(self=self))
          yield
          self.writeln('}')
docs: Set up a graphviz graph context.