Function x_axis_label_at 

pub fn x_axis_label_at(
    v: f64,
    kind: XAxisTemporalKind,
    bounds: (f64, f64),
    level: usize,
    numbers: &AxisFormat,
) -> Option<String>

An x tick at detail level (0 fullest), None past the shortest form. Narrow axes step down until labels fit: a date to year-month then year (or month-day when bounds fall in one year); a datetime to its date (or the minute within one day); a time to the minute. Numbers, and times out of range, use numbers.