Add FilledArea plot item to fill between two lines

[Eshaan-byte]

Summary

Adds a new FilledArea plot item that fills the area between two lines, useful for visualizing confidence intervals, ranges, and uncertainty bands.

Changes

• Added FilledArea struct in egui_plot/src/items/filled_area.rs
• Simple API: FilledArea::new(name, &xs, &ys_min, &ys_max) takes plain f64 slices
• Supports customizable fill color (Color32)
• Optional stroke around boundaries
• Follows existing plot item patterns

Example Usage

let xs = vec![0.0, 1.0, 2.0, 3.0];
let ys_min = vec![1.0, 1.5, 2.0, 1.8];
let ys_max = vec![3.0, 3.5, 4.0, 3.8];

let filled_area = FilledArea::new("Range", &xs, &ys_min, &ys_max)
    .fill_color(Color32::from_rgba_unmultiplied(100, 200, 100, 50));

plot_ui.add(filled_area);

[michalsustr]

Great, thank you for the PR! This is something I wanted to have in the library soon, good timing :)

Can you please add a demo example with controls? E.g.

• Copy an existing example, like box_plot as a new filled_area example.
• Plot some line, like sin(x)
• Add a control to adjust upper and lower bounds, i.e. sin(x)-delta_lb and sin(x) + delta_ub
• Add reference to demo gallery.

Thank you!

[michalsustr]

Great, please also expose pub use crate::items::FilledArea; in lib.rs

The demo uses filled polygons, which connect in a weird way to first point I presume. Please fix it (there should be three sin waves in total: sin and upper/lower bounds) and we can merge.

[michalsustr]

Oh and please run UPDATE_SNAPSHOTS=1 cargo test -p filled_area after the fix, to add new screenshots.

[michalsustr]

You might find https://github.com/0xb-s/egui_plot/pull/2/files#diff-7f61f3f9c96e213424b0aab7f15a33796acdb63d36548c771759f13030cb986a useful.

[michalsustr]

There are some CI failures, pls fix them.

[michalsustr]

Closing in favor of #202 with some fixes.