code: function brush() {
        var actives = dimensions.filter(function(p) { return !y[p].brush.empty(); }),
            extents = actives.map(function(p) { return y[p].brush.extent(); }),
            selectedPaths = foregroundPaths.filter(function(d) {
              return actives.every(function(p, i) {
                return extents[i][0] <= d[p] && d[p] <= extents[i][1];
              });
            });
        foregroundPaths.style('display', 'none');
        selectedPaths.style('display', null);
        model.selectedData = selectedPaths.data();
      }
docs: Handles a brush event, toggling the display of foreground lines.