Nannou with overlapping circles

use nannou::prelude::*;

fn main() {
    nannou::sketch(view).run();
}

fn view(app: &App, frame: Frame) {
    let columns = 5.0;
    let rows    = 4.0;
    let draw = app.draw();
    let win = app.window_rect();
    let x_step = (win.w() / columns) as usize;
    let y_step = (win.h() / rows) as usize;
    let stroke_w = win.w() / 50.0;
    let x_min = win.left() as isize;
    let x_max = win.right() as isize;
    let y_min = win.top() as isize;
    let y_max = win.bottom() as isize;
    app.set_loop_mode(LoopMode::rate_fps(60.0));
//    draw.background().color(rgba(0.0, 0.0, 0.0, 0.0));
    draw.rect().rgba(0.0, 0.0, 0.0, 0.25).w(win.w()).h(win.h());

    let radius = (app.time / 5.0).sin().abs() * 300.0 ;
    let color_yellow = rgba(1.0, 1.0, 0.0, 0.25);
    let color_cyan = rgba(0.0, 1.0, 1.0, 0.25);
    let color_magenta= rgba(1.0, 0.0, 1.0, 0.25);
    for x in (x_min..=x_max).step_by(x_step) {
        for y in (y_max..=y_min).step_by(y_step) {

            draw.ellipse()
                .x_y(x as f32, y as f32)
                .radius(radius )
                .stroke(color_yellow)
                .stroke_weight(stroke_w)
                .no_fill();

            draw.ellipse()
                .x_y(x as f32, y as f32)
                .radius(radius * 2.0 )
                .stroke(color_cyan)
                .stroke_weight(stroke_w)
                .no_fill();

            draw.ellipse()
                .x_y(x as f32, y as f32)
                .radius(radius * 4.0 )
                .stroke(color_magenta)
                .stroke_weight(stroke_w)
                .no_fill();
            }
    }
    draw.to_frame(app, &frame).unwrap()
}