title: What is the correct argument for d3.brushSelection()?
body: <p>I'm trying to get the selection values for a 1-dimensional brush, but I'm having trouble understanding what the argument for d3.brushSelection() should be. I see that the documentation says that the argument should be a node, but I don't know exactly what it means. Is it supposed to be the specific HTML element on which the brush is called on, or the svg element that holds the brush? I have tried both and both return null.</p>
  
  <pre><code>var xBrush = d3.brushX()
    .extent([[0,0], [xWidth,xHeight]])
    .on("brush", brushed);
  
  xChart.append("g")
    .attr("class", "brush")
    .call(xBrush);
  </code></pre>
  
  <p>If this is how I create my brush, how could I get the value of the selection? Thanks.</p>