Drawing a line at the center of the selected category band, on the X axis

HTML

<link rel="stylesheet" href="http://www.webdetails.pt/ctools/charts/lib/tipsy.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/require.js/2.1.19/require.min.js"></script>
<script src="//webdetails.github.io/ccc/charts/lib/amd/require.config.js"></script>
<script src="//webdetails.github.io/ccc/charts/lib/q01-01.js"></script>
<div id="cccExample" />

JavaScript

require([
  "ccc/pvc", 
  "ccc/def", 
  "ccc/protovis"
], function(pvc, def, pv) {

    var selectedCateg = "2011-07-03";

    new pvc.BarChart({
        canvas: "cccExample",
        width:  700,
        height: 400,
        axisGrid:    true,
      //  orthoAxisOffset: 0.05,
        extensionPoints: {
          plot_add: function() {
                // Shadow Panel
                return new pv.Rule()
                    .left(function() {
                        var baseScale = 
                              this.getContext().chart.axes.base.scale;
                        return baseScale(selectedCateg);
                    })
                    .top(0)
                    .bottom(0)
                    .height(null) // clear any inherited value
                    .width(null)  // clear any inherited value
                    .fillStyle('gray');
            }
        }
    })
    .setData(relational_01, {crosstabMode: false })
    .render();
});