Added an example to save plot to image

examples/save_plot/Cargo.toml

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+[package]
+name = "save_plot"
+version = "0.1.0"
+authors = ["hacknus <l_stoeckli@bluewin.ch>"]
+license = "MIT OR Apache-2.0"
+edition = "2021"
+rust-version = "1.65"
+publish = false
+
+[dependencies]
+eframe = { path = "../../crates/eframe", features = [
+    "__screenshot", # __screenshot is so we can dump a screenshot using EFRAME_SCREENSHOT_TO
+] }
+tracing-subscriber = "0.3"
+rfd = "0.11.0"
+image = { version = "0.24", default-features = false, features = ["png"] }

examples/save_plot/README.md

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+This example shows that you can save a plot in egui as a png.
+
+```sh
+cargo run -p save_plot
+```

examples/save_plot/src/main.rs

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+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::egui;
+use eframe::egui::plot::{Legend, Line, Plot, PlotPoints};
+use eframe::egui::ColorImage;
+
+fn main() -> Result<(), eframe::Error> {
+    // Log to stdout (if you run with `RUST_LOG=debug`).
+    tracing_subscriber::fmt::init();
+
+    let options = eframe::NativeOptions {
+        initial_window_size: Some(egui::vec2(350.0, 400.0)),
+        ..Default::default()
+    };
+    eframe::run_native(
+        "My egui App with a plot",
+        options,
+        Box::new(|_cc| Box::new(MyApp::default())),
+    )
+}
+
+struct MyApp {
+    /// Unit: fraction of full screen
+    plot_location: egui::Rect,
+    screenshot: Option<ColorImage>,
+}
+
+impl Default for MyApp {
+    fn default() -> Self {
+        Self {
+            plot_location: egui::Rect::EVERYTHING,
+            screenshot: None,
+        }
+    }
+}
+
+impl eframe::App for MyApp {
+    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            // these are just some dummy variables for the example
+            let height = 200.0;
+            let border_x = 11.0;
+            let border_y = 18.0;
+            let width = 300.0;
+
+            // get the size of the window
+            let window_width = ui.available_size().x;
+            let window_height = ui.available_size().y;
+
+            ui.heading("My egui Application");
+
+            // add some whitespace in y direction
+            ui.add_space(border_y);
+
+            if ui.button("Save Plot").clicked() {
+                frame.request_screenshot();
+            }
+
+            // add some whitespace in y direction
+            ui.add_space(border_y);
+
+            // this needs to be outside of the ui.horizontal()
+            let plot_location_y = window_height - ui.available_size().y;
+            ui.horizontal(|ui| {
+                // add some whitespace in x direction
+                ui.add_space(border_x);
+
+                // obviously this needs to be after the last ui.add_space
+                let plot_location_x = window_width - ui.available_size().x;
+
+                // lets set the relative plot location for plot saving purposes
+                self.plot_location = egui::Rect::from_two_pos(
+                    egui::Pos2 {
+                        x: plot_location_x / window_width,
+                        y: plot_location_y / window_height,
+                    },
+                    egui::Pos2 {
+                        x: (plot_location_x + width) / window_width,
+                        y: (plot_location_y + height) / window_height,
+                    },
+                );
+                let my_plot = Plot::new("My Plot")
+                    .height(height)
+                    .width(width)
+                    .legend(Legend::default());
+
+                // let's create a dummy line in the plot
+                let graph: Vec<[f64; 2]> = vec![[0.0, 1.0], [2.0, 3.0], [3.0, 2.0]];
+                my_plot.show(ui, |plot_ui| {
+                    plot_ui.line(Line::new(PlotPoints::from(graph)).name("curve"));
+                });
+            });
+
+            // add some whitespace in y direction
+            ui.add_space(border_y);
+        });
+
+        if let Some(screenshot) = self.screenshot.take() {
+            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
+
+                // we need to use relative coordinates since the plot location comes
+                // in relative coordinates.
+                let screenshot_width = screenshot.size[0] as f32;
+                let screenshot_height = screenshot.size[1] as f32;
+                let region = egui::Rect::from_two_pos(
+                    egui::Pos2 {
+                        x: self.plot_location.min.x * screenshot_width,
+                        y: self.plot_location.min.y * screenshot_height,
+                    },
+                    egui::Pos2 {
+                        x: self.plot_location.max.x * screenshot_width,
+                        y: self.plot_location.max.y * screenshot_height,
+                    },
+                );
+
+                // only select the plot region from the screenshot
+                // since we scale it by screenshot.size, we do not need pixels_per_point,
+                // thus i can be set to 1.0
+                let plot = screenshot.region(&region, Some(1.0));
+
+                // save the plot to png
+                image::save_buffer(
+                    &path,
+                    plot.as_raw(),
+                    plot.width() as u32,
+                    plot.height() as u32,
+                    image::ColorType::Rgba8,
+                )
+                .unwrap();
+            }
+        }
+    }
+
+    fn post_rendering(&mut self, _screen_size_px: [u32; 2], frame: &eframe::Frame) {
+        // this is inspired by the Egui screenshot example
+        if let Some(screenshot) = frame.screenshot() {
+            self.screenshot = Some(screenshot);
+        }
+    }
+}