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 level of detail, 0 the fullest, or None past the shortest form. A narrow axis steps down until its labels fit: a date to year-month and then the year, or to month-day when both ends of the axis, bounds, fall in one year; a datetime first to its date, or to the minute when the axis spans one day; a time to the minute. A number, or a time past what it can stand for, is written in numbers.