I am writing a function to convert string dates in a vector to pixel locations on a canvas. I initially achieved this via a series of for loops. But I would really like to be able to do this via an iterator with a series of filter_maps and maps as required. I think I have most of it figured out but in the first map (duration) I cannot figure out how to divide every element in the iterator, including the last element, by the last element. Any assistance will be appreciated.

pub fn line_graph_x_data() -> Result<Vec<f64>, CustomError> {

    let date_vec = [
        "2026-02-02".to_string(),
        "2026-03-14".to_string(),
        "2026-03-29".to_string(),
        "2026-04-09".to_string(),
        "2026-05-22".to_string(),
        "2026-07-15".to_string(),
    ];

    // 1) convert strings to naive dates
    // 2) Determine number of days between each date and first date ("2026-02-02") in the date_vec
    // 3) Determine number of days between top and bottom dates (earliest to most recent)
    // 4) Apply formula (((each_value - min_vallue)/(max_value - min_value)) * 600) + 100) and round

// working code below #################

    // let mut navidate_vec: Vec<NaiveDate> = Vec::new();

    // for i in 0..date_vec.len() {
    //     let nav_date = NaiveDate::parse_from_str(&date_vec[i], "%Y-%m-%d")?;
    //     navidate_vec.push(nav_date);
    // }

    // let mut numerator_vec = Vec::new();

    // for i in 0..navidate_vec.len() {
    //     let day_delta = navidate_vec[i].signed_duration_since(navidate_vec[0]).num_days();
    //     numerator_vec.push(day_delta);
    // }

    // let denominator = numerator_vec[numerator_vec.len()-1];

    // let mut x_axis_pixels = Vec::new();

    // for i in 0..numerator_vec.len() {
    //     let num_pixels = (((numerator_vec[i] as f64 / denominator as f64)) * 600.0) + 100.0;
    //     x_axis_pixels.push(num_pixels.round());
    // }

    // Ok(x_axis_pixels)

// working code above #################

    let x_axis = date_vec.iter()
        .filter_map(|item| {
            let nav_date = NaiveDate::parse_from_str(&item, "%Y-%m-%d").ok();
            return nav_date
        })
        .filter_map(|nav_date| {
            let starting_date = NaiveDate::parse_from_str(&date_vec[0], "%Y-%m-%d").ok().expect("error");
            let num_days = nav_date.signed_duration_since(starting_date)
            .num_days();
            return Some(num_days)
        })
        .map(|duration| {
            let ratio = duration as f64 / last duration item as f64 ???????
            return ratio
        })
        .map(|ratio| {
            let x_values = ((ratio * 600.0) + 100.0).round();
            return x_values
        })
        .collect::<Vec<f64>>();

}

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