colsLabel (labelFunction) {
        if (!arguments.length) {
            return this._colsLabel;
        }
        this._colsLabel = labelFunction;
        return this;
    }
Set or get the column label function. The chart class uses this function to render
column labels on the X axis. It is passed the column name.
@example
// the default label function just returns the name
chart.colsLabel(function(d) { return d; });
@param  {Function} [labelFunction=function(d) { return d; }]
@returns {Function|HeatMap}