def conversations(self, getsrcdst=None, draw = True, **kargs):
        """Graphes a conversations between sources and destinations and display it
        (using graphviz)
        getsrcdst: a function that takes an element of the list and return the source and dest
                   by defaults, return source and destination IP
        if networkx library is available returns a DiGraph, or draws it if draw = True otherwise graphviz is used
        format: output type (svg, ps, gif, jpg, etc.), passed to dot's "-T" option
        target: output filename. If None, matplotlib is used to display
        prog: which graphviz program to use"""
        if getsrcdst is None:
            getsrcdst = lambda x:(x['IP'].src, x['IP'].dst)
        conv = {}
        for p in self.res:
            p = self._elt2pkt(p)
            try:
                c = getsrcdst(p)
            except:
                #XXX warning()
                continue
            conv[c] = conv.get(c,0)+1

        if NETWORKX: # networkx is available
            gr = nx.DiGraph()
            for s,d in conv:
                if s not in gr:
                    gr.add_node(s)
                if d not in gr:
                    gr.add_node(d)
                gr.add_edge(s, d)
            if draw:
                return do_graph(gr, **kargs)
            else:
                return gr
        else:
            gr = 'digraph "conv" {\n'
            for s,d in conv:
                gr += '\t "%s" -> "%s"\n' % (s,d)
            gr += "}\n"        
            return do_graph(gr, **kargs)
Graphes a conversations between sources and destinations and display it
        (using graphviz)
        getsrcdst: a function that takes an element of the list and return the source and dest
                   by defaults, return source and destination IP
        if networkx library is available returns a DiGraph, or draws it if draw = True otherwise graphviz is used
        format: output type (svg, ps, gif, jpg, etc.), passed to dot's "-T" option
        target: output filename. If None, matplotlib is used to display
        prog: which graphviz program to use