Function prepare_bar_counts 

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

Prepare a bar chart of how many rows each category has. The counts are exact: the whole view is counted, whatever the sample size, in one streamed pass that keeps a count per category and no rows. When the rows held are the whole view, those are counted instead. Past COUNT_CATEGORY_CAP categories the count stops, and says so rather than drawing part of the view as the whole.