Function prepare_kde_by 

pub fn prepare_kde_by(
    lf: &LazyFrame,
    column: &str,
    bandwidth_factor: f64,
    range: ValueRange,
    color: ColorSplit<'_>,
    sampling: &ChartSampling,
) -> Result<KdeData>

Prepare one density curve of column per group of color. The range is the whole column’s, so every curve is clipped alike.