tstf(x, tf, _id) {
        let ts = this.env.tss[_id]
        if (!ts) {
            ts = this.env.tss[_id] = [x]
            ts.__id__ = _id
            ts.__tf__ = u.tf_from_str(tf)
            ts.__fn__ = Sampler('close').bind(ts)
        } else {
            ts.__fn__(x)
        }
        return ts
    }
Creates a new time-series & records each x.
Uses Sampler to aggregate the values
Return the an array. Id is auto-genrated
@param {*} x - A variable to sample from
@param {(number|string)} tf - Timeframe in ms or as a string
@return {TS} - New time-series