Dojox charting, problem with Columns chart type

Question

I can't figure out what's wrong. Using the latest everything from dojotoolkit.org. My chart renders fine for a Marker or Line chart:

        this.chartVol.addPlot("default", {
            type: "MarkersOnly",
            lines: false,
            markers: false,
            gap: 1,
            hAxis: 'volX',
            vAxis: 'volY'
        });

but when I change to "Columns" I get nothing.

        this.chartVol.addPlot("default", {
            type: "Columns",
            lines: false,
            markers: false,
            gap: 1,
            hAxis: 'volX',
            vAxis: 'volY'

        });

I'm looking at this howto page, it's not clear what I'm doing wrong.

http://www.dojotoolkit.org/reference-guide/dojox/charting.html#dojox-charting