Implement virtual element to allow elements to escape from auto bounds calculation

[bombless]

• Closes #143
• [ ✓ ] I have followed the instructions in the PR template

[bombless]

With the patch in this PR, issue #143 will be solved as

struct App;

impl eframe::App for App {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, |ui| {
            let plot = Plot::new("mouseover test")
                .allow_scroll(true)
                .allow_zoom(true)
                .allow_boxed_zoom(true)
                .allow_drag(true)
                .include_x(0.0)
                .include_x(0.1);

            plot.show(ui, |plot_ui| {
                let bounds = plot_ui.plot_bounds();
                let val = bounds.min()[0];

                plot_ui.text(PlotText::new(
                    "",
/* ============ no need to clamp now since we can call `.overlay(true)` */
                    PlotPoint { x: val, y: 0.0 },
                    val.to_string(),
                ).overlay(true));
            });
        });
    }
}

and to those two cases in its comments:

struct App;

impl App {


    fn draw_plot(ui: &mut egui::Ui) {
        let plot = Plot::new("dominant_freq_plot")
            .allow_scroll(false)
            .allow_zoom(true)
            .allow_boxed_zoom(true)
            .allow_drag(true)
            .default_x_bounds(0.0, 60.0)
            .default_y_bounds(-100.0, 100.0)
            .auto_bounds(Vec2b::new(true, true));

        plot.show(ui, |plot_ui| {
            // ...


            
            if let Some(pointer) = plot_ui.pointer_coordinate() {
                let txt = format!("{pointer:?}");
                plot_ui.text(
/* ============ no need to clamp now since we can call `.overlay(true)` */
                    PlotText::new("鼠标坐标提示", PlotPoint {x: pointer.x, y: pointer.y}, txt)
                        .overlay(true)
                        .anchor(Align2([Align::Min, Align::Max]))
                );
            }
        });
    }
}

struct App;

// Constants for time and frequency axis bounds
const TIME_BOUNDS: (f64, f64) = (0.0, 10.0);
const FREQ_BOUNDS: (f64, f64) = (0.0, 10.0);

impl App {
    fn draw_plot(&self, ui: &mut egui::Ui) {
        // Create a plot with specified interaction settings
        let plot = Plot::new("dominant_freq_plot")
            .allow_scroll(false)
            .allow_zoom(true)
            .allow_boxed_zoom(true)
            .allow_drag(true)
            .default_x_bounds(TIME_BOUNDS.0, TIME_BOUNDS.1)
            .default_y_bounds(FREQ_BOUNDS.0, FREQ_BOUNDS.1)
            .auto_bounds(Vec2b::new(true, true));

        plot.show(ui, |plot_ui| {
            let beat_duration = 1f64;
            let bounds = plot_ui.plot_bounds();
            let y_span = bounds.max()[1] - bounds.min()[1];

            // Rectangle position: at the top of the plot
            let rect_y_center = bounds.max()[1] - 0.05 * y_span;
            let rect_height = 0.08 * y_span;
            let rect_width = beat_duration * 0.8;

            // Rectangle corners
            let rect_x_min = 0.0f64;
            let rect_x_max = 0.0 + rect_width;
            let rect_y_max = rect_y_center + rect_height / 2.0;

/* ============ no need to clamp now since we can call `.overlay(true)` */

            // let rect_x_min = rect_x_min.clamp(TIME_BOUNDS.0, TIME_BOUNDS.1);
            // let rect_x_max = rect_x_max.clamp(TIME_BOUNDS.0, TIME_BOUNDS.1);
            // let rect_y_max = rect_y_max.clamp(FREQ_BOUNDS.0, FREQ_BOUNDS.1);

            println!("{:?}", vec![
                [rect_x_min, rect_y_max],
                [rect_x_max, rect_y_max],
            ]);
            // Draw top edge of the rectangle
            plot_ui.line(
                Line::new(
                    "PlotPoints",
                    PlotPoints::from_iter(vec![
                        [rect_x_min, rect_y_max],
                        [rect_x_max, rect_y_max],
                    ]),
                )
                    .overlay(true)
                    .width(10.0),
            );
        });
    }
}