Improve behavior of plot auto-bounds with reduced data

[abey79]

• Fixes "Double-click to reset view" for single-point plots doesn't work #3808
• Fixes Plot bounds broken when all y values are the same #2307

This PR improves the behaviour of auto-bounds with data that:

• is a single point
• where all X values are the same (e.g. vertical line)
• where all Y values are the same (e.g. horizontal line)

In all case, the auto-bound now aim to center on the data. For span, when available, it use the same as the other axis. If the data range of the other axis is also degenerate, then it defaults to +/- 1.0.

better_plot_bounds.mp4

Test code

#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
#![allow(rustdoc::missing_crate_level_docs)] // it's an example

use eframe::egui;
use egui_plot::{Legend, Line, Plot, PlotPoints, Points};

fn main() -> Result<(), eframe::Error> {
    env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).

    let options = eframe::NativeOptions {
        viewport: egui::ViewportBuilder::default().with_inner_size([350.0, 200.0]),
        ..Default::default()
    };
    eframe::run_native(
        "My egui App with a plot",
        options,
        Box::new(|_cc| Ok(Box::<MyApp>::default())),
    )
}

#[derive(Default)]
struct MyApp {}

impl eframe::App for MyApp {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
        let mut plot_rect = None;
        egui::CentralPanel::default().show(ctx, |ui| {
            if ui.button("Save Plot").clicked() {
                ctx.send_viewport_cmd(egui::ViewportCommand::Screenshot);
            }

            let my_plot = Plot::new("My Plot").legend(Legend::default());

            // let's create a dummy line in the plot
            let inner = my_plot.show(ui, |plot_ui| {
                plot_ui.line(
                    Line::new(PlotPoints::from(vec![
                        [0.0, 10.0],
                        [2.0, 10.0],
                        [3.0, 10.0],
                    ]))
                    .name("y = 10.0"),
                );

                plot_ui.line(
                    Line::new(PlotPoints::from(vec![
                        [10.0, 10.0],
                        [10.0, 11.0],
                        [10.0, 12.0],
                    ]))
                    .name("x = 10.0"),
                );
                plot_ui.points(
                    Points::new(PlotPoints::from(vec![[5.0, 5.0]]))
                        .name("(5,5)")
                        .radius(3.0),
                );
                plot_ui.points(
                    Points::new(PlotPoints::from(vec![[5.0, 7.0]]))
                        .name("(5,7)")
                        .radius(3.0),
                );
            });
            // Remember the position of the plot
            plot_rect = Some(inner.response.rect);
        });

        // Check for returned screenshot:
        let screenshot = ctx.input(|i| {
            for event in &i.raw.events {
                if let egui::Event::Screenshot { image, .. } = event {
                    return Some(image.clone());
                }
            }
            None
        });

        if let (Some(screenshot), Some(plot_location)) = (screenshot, plot_rect) {
            if let Some(mut path) = rfd::FileDialog::new().save_file() {
                path.set_extension("png");

                // for a full size application, we should put this in a different thread,
                // so that the GUI doesn't lag during saving

                let pixels_per_point = ctx.pixels_per_point();
                let plot = screenshot.region(&plot_location, Some(pixels_per_point));
                // save the plot to png
                image::save_buffer(
                    &path,
                    plot.as_raw(),
                    plot.width() as u32,
                    plot.height() as u32,
                    image::ColorType::Rgba8,
                )
                .unwrap();
                eprintln!("Image saved to {path:?}.");
            }
        }
    }
}

[emilk]

alternative 🤷

Suggested change

	pub fn new(frame: Rect, bounds: PlotBounds, x_centered: bool, y_centered: bool) -> Self {
	let mut new_bounds = bounds;
	pub fn new(frame: Rect, mut bounds: PlotBounds, x_centered: bool, y_centered: bool) -> Self {

[abey79]

Nope! 😱 I need the original, unmodified bounds throughout, otherwise the modifications for the X axis interact badly with the modifications for the Y axis. Took me the longest time to figure that out.

[abey79]

ill add a comment to that effect