Edit in JSFiddle

function barChartPlotter(e) {
  var ctx = e.drawingContext;
  var points = e.points;
  var y_bottom = e.dygraph.toDomYCoord(0);  // see <a href="http://dygraphs.com/jsdoc/symbols/Dygraph.html#toDomYCoord">jsdoc</a>

  // This should really be based on the minimum gap
  var bar_width = 2/3 * (points[1].canvasx - points[0].canvasx);
  ctx.fillStyle = e.color;  // a lighter shade might be more aesthetically pleasing

  // Do the actual plotting.
  for (var i = 0; i < points.length; i++) {
    var p = points[i];
    var center_x = p.canvasx;  // center of the bar

    ctx.fillRect(center_x - bar_width / 2, p.canvasy, bar_width, y_bottom - p.canvasy);
    ctx.strokeRect(center_x - bar_width / 2, p.canvasy, bar_width, y_bottom - p.canvasy);
  }
}

g = new Dygraph(document.getElementById("graph"),
                 "X,Y\n" +
                 "1,2\n" +
                 "2,4\n" +
                 "3,6\n" +
                 "4,8\n" +
                 "5,10\n" +
                 "6,12\n" +
                 "7,14\n" +
                 "8,16\n",
                 {
                     // options go here. See http://dygraphs.com/options.html
                     legend: 'always',
                     animatedZooms: true,
                     plotter: barChartPlotter,
                     dateWindow: [0, 9]
                 });
                 

<!-- You may set style: "width: ...; height: ..." to size the chart -->
<div id="graph"></div>

/*
Relevant CSS classes include:

Labels on the X & Y axes:
.dygraph-axis-label
.dygraph-axis-label-x
.dygraph-axis-label-y
.dygraph-axis-label-y2

Chart labels, see http://dygraphs.com/tests/styled-chart-labels.html
.dygraph-title
.dygraph-xlabel
.dygraph-ylabel
.dygraph-y2label

The legend, see http://dygraphs.com/tests/series-highlight.html
.dygraph-legend
*/
.dygraph-title {
    color: gray;
}