Function prepare_bar_aggregate 

pub fn prepare_bar_aggregate(
    lf: &LazyFrame,
    schema: &Schema,
    spec: &BarAggregate<'_>,
    sampling: &ChartSampling,
) -> Result<BarData>

A bar chart of value aggregated per category (and color group) in one lazy group-by; a count needs no value column, and uncolored is the exact count chart.