d3js: Use view port center for zooming focal point

Question

We are trying to implement zoom buttons on top of a map created in D3 - essentially as it works on Google maps. The zoom event can be dispatched programmatically using

Answer 1

A more succinct version of Wil's solution:

  var vis = d3.select('.vis');
  var zoom = d3.behavior.zoom()...
  var width = .., height = ..;

  function zoomByFactor(factor) {
    var scale = zoom.scale();
    var extent = zoom.scaleExtent();
    var newScale = scale * factor;
    if (extent[0] <= newScale && newScale <= extent[1]) {
      var t = zoom.translate();
      var c = [width / 2, height / 2];
      zoom
        .scale(newScale)
        .translate(
          [c[0] + (t[0] - c[0]) / scale * newScale, 
           c[1] + (t[1] - c[1]) / scale * newScale])
        .event(vis.transition().duration(350));
    }
  };

  function zoomIn() { zoomByFactor(1.2); }
  function zoomOut() { zoomByFactor(0.8); }