function toFixed(val) {
            return val === dataBound[0]
                ? 'min'
                : val === dataBound[1]
                ? 'max'
                : (+val).toFixed(Math.min(precision, 20));
        }
FIXME
Do not publish to thirt-part-dev temporarily
util the interface is stable. (Should it return
a function but not visual meta?)

@pubilc
@abstract
@param {Function} getColorVisual
       params: value, valueState
       return: color
@return {Object} visualMeta
       should includes {stops, outerColors}
       outerColor means [colorBeyondMinValue, colorBeyondMaxValue]