I have a time-series in pandas with several products (id's: a, b, etc), but with monthly holes. I have to fill those holes. It may be with np.nan or any other constant. I tried groupby but I wasnt able.

```
date          id   units
2022-01-01    a     10
2022-01-01    b     100
2022-02-01    a     15
2022-03-01    a     30
2022-03-01    b     70
2022-05-01    b     60
2022-06-01    a     8
2022-06-01    b     90

```

Should be:

```
date          id   units
2022-01-01    a     10
2022-01-01    b     100
2022-02-01    a     15
2022-02-01    b     np.nan
2022-03-01    a     30
2022-03-01    b     70
2022-04-01    a     np.nan
2022-04-01    b     np.nan
2022-05-01    a     np.nan
2022-05-01    b     60
2022-06-01    a     8
2022-06-01    b     90

```