Function label_number_marks 

pub fn label_number_marks(
    accuracy: Option<f64>,
    prefix: &str,
    suffix: &str,
    scale: f64,
    decimal_mark: &str,
    big_mark: &str,
) -> impl Fn(f64) -> String + Send + Sync

label_number with an explicit decimal mark and thousands mark ("" disables grouping).

use ggplot_rs::scale::format::label_number_marks;
let f = label_number_marks(Some(0.01), "", " kg", 1.0, ",", ".");
assert_eq!(f(-1234.5), "-1.234,50 kg");