let data = `1684695900000   1694
1684696200000   6412
1684696500000   11708
1684696800000   18651
1684697100000   20681
1684697400000   20431
1684697700000   21725
1684698000000   19283
1684698300000   19628
1684698600000   19387
1684698900000   19128
1684699200000   20321
1684699500000   20032
1684699800000   20528
1684700100000   20823
1684700400000   20393`.split('\n').map(e => {
    let [timestamp, viewers] = e.split('\t').map(n => parseInt(n));
    return {timestamp: timestamp, viewers: viewers}
  })

let total = 0, last, first, avg
if(data.length > 1) {
  for(const d of data) {
    if(last) {
      total += (last.viewers + d.viewers)/2*(d.timestamp-last.timestamp);
    } else first = d;
    last = d;
  }
  avg = Math.round(total/(last.timestamp - first.timestamp));
} else {
  first = data[0];
  last = data[0];
  avg = data[0].viewers;
}

console.log(avg)