title: how to apply Linear gradient in Node
body: <p>I'm still new in D3.js, here some question that need help.
  May I ask about how to apply Linear gradient in Node?
  I had tried a few time of changes, but its not work.</p>
  
  <pre><code>  // draw nodes
      var node = svg.selectAll(".node")
        .data(force.nodes())
        .enter().append("g")
        .attr("class", "node")
  
       .style("fill", function(d) { 
       return color(d.group); })
  
        .style("opacity", options.opacity)
        .on("mouseover", mouseover)
  
        .on("mouseout", mouseout)
        .on("click", click)
        .call(drag);
  
  
      node.append("circle")
        .attr("r", function(d){return nodeSize(d);})
        .style("stroke", "#fff")
        .style("opacity", options.opacity)
        .style("stroke-width", "1.5px");
  
  //Linear Gradient
  node.append("stop")
  .attr("offset","0%)
  .style("stop-color","white");
  
  
  node.append("stop")
  .attr("offset","100%")
  .style("style-color",function(d){return color(d.group);});
  
  </code></pre>