How do I get the first, last occurrence (>0) of a particular column in a time series
df,

shape: (4, 3)
┌────────────┬────────────┬────────────┐
│ date       ┆ column_one ┆ column_two │
│ ---        ┆ ---        ┆ ---        │
│ date       ┆ f64        ┆ i64        │
╞════════════╪════════════╪════════════╡
│ 2024-06-01 ┆ 0.0        ┆ 0          │
│ 2024-06-02 ┆ 0.0        ┆ 1          │
│ 2024-06-03 ┆ 1.0        ┆ 2          │
│ 2024-06-04 ┆ 1.2        ┆ 3          │
└────────────┴────────────┴────────────┘

and keep it stored in a separate df,

columns | first_appearance | last_appearance
column_one | 2024-06-03 | 2024-06-04
column_two | 2024-06-02 | 2024-06-04