Function prepare_bar_data 

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

Prepare a bar chart: one bar per row, the category from category, the length from value. The chart takes a grouped result — one row per category — and refuses a category that repeats rather than guess how to combine its rows. A null category is a bar of its own; a null value leaves its category out, counted in no_value.