vwma(src, len, _id) {
        let id = this._tsid(_id, `vwma(${len})`)
        let vol = this.env.shared.vol
        let sxv = this.ts(src[0] * vol[0], id+'1', src.__tf__)
        let res =
            this.sma(sxv, len, id+'2')[0] /
            this.sma(vol, len, id+'3')[0]
        return this.ts(res, id+'4', src.__tf__)
    }
Volume Weighted Moving Average
@param {TS} src - Input
@param {number} len - length
@return {TS} - New time-series