title: Adding point and value to line chart... how to color the point like the line?
body: <p>I've started from the multiline example <a href="http://bl.ocks.org/mbostock/3884955">http://bl.ocks.org/mbostock/3884955</a>
  I extended it to show the dots along the line, but I'm not able to give to the circle the same color of the line...
  I'm really new in d3.js and I really need an advice.
  here the example page: <a href="http://www.danielepennati.com/d3/linea.html">http://www.danielepennati.com/d3/linea.html</a>
  I've change some variable name to make the script more general so there is some difference with the original example code. the main one is the name of the variable that contain the mapped data: it is "column" and not "cities"</p>
  
  <pre><code>d3.tsv(surce_data, function(error, data) {
  color.domain(d3.keys(data[0]).filter(function(key) { return key !== "id"; }));
  
  var column = color.domain().map(function(name) {
    return {
      name: name,
      values: data.map(function(d) {
        return {id: d.id, value: +replace(d[name])};
      })
    };
  });
  </code></pre>
  
  <p>the second main difference is the x axsis: in my code it is ordinal and not linear.
  so to draw the line the code is: </p>
  
  <pre><code>var tracciato = svg.selectAll(".line-group")
  .data(column)
  .enter().append("g")
  .attr("class", "line-group");
  
  tracciato.append("path")
  .attr("class", "line")
  .attr("d", function(d) { return line(d.values); })
  .style("stroke", function(d) { return color(d.name); });
  </code></pre>
  
  <p>to make the points along the line I've wrote this code:</p>
  
  <pre><code>var point = tracciato.append("g")
      .attr("class", "line-point");
  
      point.selectAll('circle')
      .data(function(d,i){ return d.values})
      .enter().append('circle')
      .attr("cx", function(d, i) {
          return x(i) + x.rangeBand() / 2;
        })
       .attr("cy", function(d, i) { return y(d.value) })
       .attr("r", 5);
  </code></pre>
  
  <p>I'd link the points color be the same of the line, but the problem is the colors are assigned to the "column" object. I don't know how to give to each new circle within the same column the same column color...</p>
  
  <p>I don't know if my problem is clear, please ask me if you need any more specification.
  Thanks</p>