[BUG] Out-of-range data is deformed (line slopes changed) #689

Describe the bug

I'm plotting a simple test plot of the equation y = x^2.

// Simple chart setup for testing
    let root = SVGBackend::new("test.svg", (640, 480)).into_drawing_area();
    root.fill(&WHITE).unwrap();

    let mut chart = ChartBuilder::on(&root)
        .caption("Test Chart", ("sans-serif", 50).into_font())
        .x_label_area_size(30)
        .y_label_area_size(30)
        .margin(5)
        .build_cartesian_2d(0..10, 0..10)
        .unwrap();

    chart.configure_mesh().draw().unwrap();

    chart
        .draw_series(LineSeries::new(
            (0..10).map(|x| (x, x * x)),
            &RED,
        ))
        .unwrap();

    root.present().expect("Unable to write result to file");

This produces the following plot:

As you can see, the data goes out of range. What I would expect is that either:

• The line crosses outside the chart rectangle.
• The line is clipped off at the boundary
• The code produces an error/panic

However, with the current implementation, the slope of the line is instead adjusted between x=3 and x=4, and for x > 4, it just goes flat against the top boundary. I consider this a severe deformation of the input data.

Sure, in this example I artificially created some data that goes out-of-bounds, but with datasets in the wild, you might accidentally create artifacts in the plot that may lead to incorrect conclusions being drawn.

The root cause is this:

/// Used for 2d coordinate transformations.
pub struct BackendCoordOnly;

impl CoordMapper for BackendCoordOnly {
    type Output = BackendCoord;
    fn map<CT: CoordTranslate>(coord_trans: &CT, from: &CT::From, rect: &Rect) -> BackendCoord {
        rect.truncate(coord_trans.translate(from))
    }
}

Unfortunately, the truncation behavior here is a bit naïve, as this also applies to the endpoints of lines.

Version Information

plotters = { version = "0.3.7", default-features = false, features = ["svg_backend", "all_series","colormaps","full_palette","histogram"] }