_chart = (
    alt.Chart([]) # <-- replace with data
    .transform_filter(f"datum.date_col != null")
    .transform_timeunit(as_="_date_col", field="date_col", timeUnit="yearmonthdate")
    .mark_area()
    .encode(
        x=alt.X("_date_col:T", title="date_col"),
        y=alt.Y("count():Q", title="Number of records"),
        tooltip=[
            alt.Tooltip("_date_col:T", title="date_col", timeUnit="yearmonthdate"),
            alt.Tooltip("count():Q", title="Number of records", format=",.0f")
        ]
    ).properties(width="container").configure_view(stroke=None)
)
_chart