context.append("g")
  .attr("class", "brush")
  .call(brush)
  .call(brush.move, [x2(new Date(2013, 0, 1)), x2(new Date(2013, 6, 1))])
  .selectAll(".overlay")
  .each(function(d) {
    d.type = "selection";
  })
  .on("mousedown touchstart", function() { d3.event.stopPropagation(); })
  .on("mousedown touchstart", brushcentered)