Running average with rxjs 5

I would like to observe rolling averages with rxjs ^5

Half-solution

const data$ = Rx.Observable.range(1, 9);
const dataToAverage$ = data$.bufferCount(4, 1);
const movingAverage$ = dataToAverage$.map(arr =>
                       arr.reduce((acc, cur) => acc + cur) / arr.length);

• The above code works fine, except that the first data array it averages is 1,2,3,4.
• How could I average 1 and 1,2 and 1,2,3 as well?
• Have a play at https://jsfiddle.net/KristjanLaane/kLskp71j/