Long legend labels overflow out of the legend box

[ilya-zlobintsev]

When using series labels that are very long, at some point the legend box size stops scaling correctly with the text size, causing it to overflow.

Example chart drawn with the plotters-cairo backend:

Same chart, but with the bitmap backend (no overflow here):

I did some basic debugging, and it seems like plotters does not call estimate_text_size on the backend for the legend labels, and instead calculates text dimensions internally based on the font. Perhaps the estimated text size ends up being different from what it actually is in draw_text on the backend?

Chart code to reproduce this:

fn plot_pdf<'a, DB: DrawingBackend + 'a>(backend: DB) -> Result<(), Box<dyn Error + 'a>> {
    let root = backend.into_drawing_area();

    root.fill(&WHITE)?;

    let root_area = root.titled("Image Title", ("noto-sans", 60))?;

    let x_axis = (-3.4f32..3.4).step(0.1);

    let mut cc = ChartBuilder::on(&root_area)
        .margin(5)
        .set_all_label_area_size(50)
        .caption("Sine and Cosine", ("noto-sans", 40))
        .build_cartesian_2d(-3.4f32..3.4, -1.2f32..1.2f32)?;

    cc.configure_mesh()
        .x_labels(20)
        .y_labels(10)
        .disable_mesh()
        .x_label_formatter(&|v| format!("{:.1}", v))
        .y_label_formatter(&|v| format!("{:.1}", v))
        .draw()?;

    cc.draw_series(LineSeries::new(x_axis.values().map(|x| (x, x.sin())), &RED))?
        .label("Sine")
        .legend(|(x, y)| PathElement::new(vec![(x, y), (x + 20, y)], RED));

    cc.draw_series(LineSeries::new(
        x_axis.values().map(|x| (x, x.cos())),
        &BLUE,
    ))?
    .label("An extremely long description of what a Cosine is that overflows")
    .legend(|(x, y)| PathElement::new(vec![(x, y), (x + 20, y)], BLUE));

    cc.configure_series_labels()
        .border_style(BLACK)
        .label_font(("noto-sans", 14))
        .draw()?;

    // Otherwise you can use a function to construct your pointing element yourself
    cc.draw_series(PointSeries::of_element(
        (-3.0f32..2.1f32).step(1.0).values().map(|x| (x, x.sin())),
        5,
        ShapeStyle::from(&RED).filled(),
        &|coord, size, style| {
            EmptyElement::at(coord)
                + Circle::new((0, 0), size, style)
                + Text::new(format!("{:?}", coord), (0, 15), ("noto-sans", 15))
        },
    ))?;

    root.present()?;
    Ok(())
}