Function prepare_bar_data 

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

A bar chart, one bar per row: category from category, length from value. Expects a grouped result and refuses a repeated category rather than guess. A null category is its own bar; a null value is left out, counted in no_value.