Function create_grouped_series 

fn create_grouped_series<'a, Outer, Inner, Series>(
    grouped_time_log: BTreeMap<impl Into<Option<&'a Outer>>, BTreeMap<impl Into<Option<&'a Inner>> + Clone, Vec<&'a TimeLog>>>,
) -> (Vec<Series>, Vec<String>)
where
    Outer: Display + 'a,
    Inner: Display + 'a,
    Series: MultiSeries,

Create a Series for all data points of a key to display in a chart. The key of the BTreeMap is Into<Option<T> to allow for Option<T> and non-Option<T> keys. If it is None, the key is “None”. Returns the Series and the axis labels for the grouped bar chart.