Function prepare_bar_aggregate 

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

A bar chart of value aggregated per category (and per color group): one lazy group-by over every row of the view. A count needs no value column; one without a color is the exact count a bar chart of counts draws.