wma(src, len, _id) {
        let id = this._tsid(_id, `wma(${len})`)
        let norm = 0
        let sum = 0
        for (var i = 0; i < len; i++) {
            let w = (len - i) * len
            norm += w
            sum += src[i] * w
        }
        return this.ts(sum / norm, id, src.__tf__)
    }
Weighted moving average
@param {TS} src - Input
@param {number} len - length
@return {TS} - New time-series