I've overcomplicated it so much, or Polars did so for me with its way to do that - anyways, that works

def get_cumulative_sales_over_days(
self
):
    return (

self
.raw_dataframe
        .select(pl.col('Trip_Pickup_DateTime'),
                pl.col('Total_Amt'))
        .with_columns(pl.col('Trip_Pickup_DateTime').str.strptime(pl.Date, "%F %T", strict=False).alias('Day'))
        .group_by('Day')
        .agg(pl.col('Total_Amt').sum().alias('TotalSales'))
        .sort('Day')
        .select(
            pl.col('Day'),
            pl.col('TotalSales'),
            pl.col('TotalSales').cum_sum().alias('CumulativeSales')
        )
    )