Function prepare_xy_by 

pub fn prepare_xy_by(
    lf: &LazyFrame,
    schema: &Schema,
    x: &str,
    y: &str,
    color: ColorSplit<'_>,
    sampling: &ChartSampling,
) -> Result<GroupedSeries>

A line or scatter chart of y split by color, from the rows a chart samples: one series per group, in X order, each breaking where its Y has no value.