Function prepare_bar_counts 

pub fn prepare_bar_counts(
    lf: &LazyFrame,
    category: &str,
    order: BarOrder,
    cap: usize,
    sampling: &ChartSampling,
) -> Result<BarData>

A bar chart of rows per category, exact: the whole view is counted in one streamed pass keeping only counts (or the held rows, if they are the whole view). Past COUNT_CATEGORY_CAP categories the count stops and says so.