Function stack_segment_bottom 

pub fn stack_segment_bottom(
    raw_series: &[ProjectedSeries],
    stacked: &[ProjectedSeries],
    stack_groups: &HashMap<String, String>,
    _series_defs: &[SeriesDef],
    series_id: &str,
    row: usize,
    offset: StackOffset,
    order: StackOrder,
) -> f64

Bottom of a stacked segment at a row (cumulative of prior series in group).