title: d3 use drop down to filter for column headers
body: <p>I have data in a csv with countries as columns headers and a timeseries of row data with date as the primary key.</p>
  
  <p>What I am attempting should be fairly straight forward but I haven't been able to get it to work. 
  I have a drop down box which contains the country column headers. Once on of these is selected I want to reference the selection to fill a chart.</p>
  
  <p>I currently have this working if I explicitly state the name of the column header i.e if the header is USA which will load all data for that column. </p>
  
  <pre><code>data1.forEach(function(d) {
                      d.Date = parseDate(d.Date);
                      d.country= d.USA;
  
                      });
  </code></pre>
  
  <p>But I need this to be based on the item selected which I can get by</p>
  
  <pre><code>var myCountry = this.options[this.selectedIndex].__data__;
  </code></pre>
  
  <p>I am just missing how to use this variable to reference the column d.USA</p>