This is my code :

legend = svg.append("g")
      .data(json.links)
      .attr("class","legend")
      .attr("transform","translate(50,30)")
      .attr("data-legend", function(d){ return d.relation; })
      .style("fill", function (d) { return color(d.group); })
      .call(d3.legend);

It's work but only one relation appears in the lengend. Can you help me ? Thanks a lot !