Calculate the min and max values for this series's xField.
@return {Array} [min, max]
function() {
        var me = this,
            chart = me.chart,
            store = chart.getChartStore(),
            data = store.data.items,
            count = me.getRecordCount(),
            i, ln, record,
            min, max,
            xField = me.xField,
            xValue;

        if (count > 0) {
            min = Infinity;
            max = -min;
                
            for (i = 0, ln = data.length; i < ln; i++) {
                record = data[i];
                xValue = record.get(xField);
                if (xValue > max) {
                    max = xValue;
                }
                if (xValue < min) {
                    min = xValue;
                }
            }
            
            // If we made no progress, treat it like a category axis
            if (min == Infinity) {
                min = 0;
            }
            
            if (max == -Infinity) {
                max = count - 1;
            }
        } else {
            min = max = 0;
        }
        return [min, max];
    }