diff_format_fn_t format_callback;
        void *format_callback_data;
        diff_prefix_fn_t output_prefix;
+       int output_prefix_length;
        void *output_prefix_data;
 };
 
 
        struct git_graph *graph = data;
        static struct strbuf msgbuf = STRBUF_INIT;
 
+       assert(opt);
        assert(graph);
 
+       opt->output_prefix_length = graph->width;
        strbuf_reset(&msgbuf);
        graph_padding_line(graph, &msgbuf);
        return &msgbuf;
         */
        opt->diffopt.output_prefix = diff_output_prefix_callback;
        opt->diffopt.output_prefix_data = graph;
+       opt->diffopt.output_prefix_length = 0;
 
        return graph;
 }