Edit in JSFiddle

var stxx = new CIQ.ChartEngine({
  container: $$$(".chartContainer")
}); // Declare a CIQ.ChartEngine object. This is the main object for drawing charts.

//override the default rendering function for the volume overly
// and customize to render the histogram any way you you want it.
CIQ.Studies.studyLibrary["vol undr"].seriesFN = function(stx, sd, quotes) {
  var seriesParam = [{
    field: "Volume",
    fill_color_up: stx.canvasStyle("stx_volume_underlay_up").color,
    border_color_up: stx.canvasStyle("stx_volume_underlay_up").borderLeftColor,
    opacity_up: stx.canvasStyle("stx_volume_underlay_up").opacity,
    fill_color_down: stx.canvasStyle("stx_volume_underlay_down").color,
    border_color_down: stx.canvasStyle("stx_volume_underlay_down").borderLeftColor,
    opacity_down: stx.canvasStyle("stx_volume_underlay_down").opacity
  }];
  var params = {
    name: "Volume",
    panel: sd.panel,
    heightPercentage: sd.inputs.HeightPercentage ? sd.inputs.HeightPercentage : sd.study.parameters.heightPercentage,
    widthFactor: .75 // add gaps between bars by customizing the widthFactor
  };
  stx.drawHistogram(params, seriesParam);
};

stxx.loadChart("SPY", sampleData, null, function() {
  // add anything you want done in the callback

  // and a volume underlay using a customized seriesFN ( above )
  CIQ.Studies.quickAddStudy(
    stxx,
    "vol undr", {}, {
      "Up Volume": "gray",
      "Down Volume": "gray",
      "Up Border": "transparent",
      "Down Border": "transparent",
    });
});
<div class="chartContainer" style="width:600px;height:400px;position:relative;"></div>

<script src="https://jsfiddle.chartiq.com/chart/examples/data/STX_SAMPLE_DAILY.js"></script>
<script src="https://jsfiddle.chartiq.com/chart/js/chartiq.js"></script>