Function prepare_box_by 

pub fn prepare_box_by(
    lf: &LazyFrame,
    column: &str,
    by: ColorSplit<'_>,
    range: ValueRange,
    sampling: &ChartSampling,
) -> Result<BoxPlotData>

Prepare one box of column per group of by: a box per category. The range is each group’s own, as each box describes its group.