title: ColorBrewer in D3 does not work
body: <p>I overlay a leaflet map with the d3 library. The points get displayed, as well as the map. However, the Colorbrewer does not work... 
  Its supposed to color the dots on the map according to their value, instead, they stay black. I could hardcode that with something like <code>if value == 0.1</code> but thats not what I want...</p>
  
  <p>Thats my code, the structure of cities.json can be seen <a href="https://raw.githubusercontent.com/Stophface/d3_leaflet/master/cities.json" rel="nofollow">here</a>, the colorbrewer is this <a href="http://bl.ocks.org/mbostock/5577023" rel="nofollow">one</a></p>
  
  <pre><code>        ...
  
                      // add colorbrewer
                      var colorScale = d3.scale.quantize()
                      .domain([extent[0], extent[1]])
                      .range(colorbrewer.YlGn[n]);
  
                      // uses d3 data join method
                      // for each data point a "path" is created 
                      var feature = g.selectAll("path")
                      .data(collection.features)
                      .enter()
                      .append("path")
                      .style("fill", function(d) {
                          colorScale(d.properties.pop_max);
                      });
  
          ...
  </code></pre>
  
  <p>Any ideas what is going wrong?!
  There are negative values in my <code>d.properties.pop_max</code>. Could that be the problem?</p>