pub trait IStatRate {
    fn reset(&self) -> Result<(), &'static str>;
    fn avg(&self) -> Option<(f64 /*period*/, f64 /*rate*/)>;
    fn max(&self) -> Option<(f64, f64)>;
    fn min(&self) -> Option<(f64, f64)>;
    fn set_window_period(&self, f64) -> Result<(), &'static str>;
}