fix: preserve paired series fill boundaries

[zao111222333]

Previously, series-to-series fills were first reduced to strictly increasing x coordinates before interpolation. This dropped duplicate x values and could flatten or remove vertical step edges, producing incorrect filled regions for step-like boundaries. It also could not represent boundaries that are naturally paired by y instead of x.

This PR updates fill generation so that:

• equal-length series with matching paired x coordinates are filled segment-by-segment
• equal-length series with matching paired y coordinates are also filled segment-by-segment
• duplicate coordinates and vertical step edges are preserved
• non-paired series still use the existing monotonic x interpolation fallback

Demo

Before:

Now:

Demo code:

//! Demonstrates fill regions between paired series boundaries.
use iced::Element;
use iced_plot::{
    Color, Fill, LineStyle, MarkerStyle, PlotUiMessage, PlotWidget, PlotWidgetBuilder, Series,
};

fn main() -> iced::Result {
    iced::application(new, update, view)
        .font(include_bytes!("fonts/FiraCodeNerdFont-Regular.ttf"))
        .default_font(iced::Font::with_name("FiraCode Nerd Font"))
        .run()
}

fn update(widget: &mut PlotWidget, message: PlotUiMessage) {
    widget.update(message);
}

fn view(widget: &PlotWidget) -> Element<'_, PlotUiMessage> {
    widget.view()
}

fn new() -> PlotWidget {
    let step_lower_positions = vec![
        [0.0, 0.0],
        [1.0, 0.0],
        [1.0, 0.7],
        [2.0, 0.7],
        [2.0, 0.25],
        [3.0, 0.25],
        [3.0, 1.05],
        [4.0, 1.05],
        [4.0, 0.55],
        [5.0, 0.55],
        [5.0, 1.35],
        [6.0, 1.35],
    ];
    let step_upper_positions: Vec<[f64; 2]> = step_lower_positions
        .iter()
        .map(|p| [p[0], p[1] + 1.05])
        .collect();

    let left_positions = vec![
        [7.0, -0.25],
        [7.7, -0.25],
        [7.7, 0.55],
        [7.2, 0.55],
        [7.2, 1.35],
        [7.9, 1.35],
        [7.9, 2.15],
        [7.4, 2.15],
    ];
    let right_positions: Vec<[f64; 2]> =
        left_positions.iter().map(|p| [p[0] + 0.9, p[1]]).collect();

    let step_lower = Series::line_only(
        step_lower_positions,
        LineStyle::solid().with_pixel_width(2.5),
    )
    .with_marker_style(MarkerStyle::circle(3.5))
    .with_color(Color::from_rgb(0.1, 0.45, 0.95))
    .with_label("step lower");
    let step_upper = Series::line_only(
        step_upper_positions,
        LineStyle::solid().with_pixel_width(2.5),
    )
    .with_marker_style(MarkerStyle::circle(3.5))
    .with_color(Color::from_rgb(0.95, 0.35, 0.1))
    .with_label("step upper");
    let step_fill = Fill::new(step_lower.id, step_upper.id)
        .with_color(Color::from_rgba(0.15, 0.6, 0.95, 0.24))
        .with_label("paired x fill");

    let left_boundary = Series::line_only(left_positions, LineStyle::solid().with_pixel_width(2.5))
        .with_marker_style(MarkerStyle::square(3.8))
        .with_color(Color::from_rgb(0.15, 0.62, 0.3))
        .with_label("left boundary");
    let right_boundary =
        Series::line_only(right_positions, LineStyle::solid().with_pixel_width(2.5))
            .with_marker_style(MarkerStyle::square(3.8))
            .with_color(Color::from_rgb(0.6, 0.28, 0.82))
            .with_label("right boundary");
    let side_fill = Fill::new(left_boundary.id, right_boundary.id)
        .with_color(Color::from_rgba(0.3, 0.75, 0.45, 0.24))
        .with_label("paired y fill");

    PlotWidgetBuilder::new()
        .with_x_lim(-0.3, 9.2)
        .with_y_lim(-0.8, 3.2)
        .with_x_label("x")
        .with_y_label("y")
        .add_series(step_lower)
        .add_series(step_upper)
        .add_series(left_boundary)
        .add_series(right_boundary)
        .add_fill(step_fill)
        .add_fill(side_fill)
        .with_cursor_overlay(true)
        .with_crosshairs(true)
        .build()
        .unwrap()
}