[BUG] `Circle` size is fixed over changing coordinate systems.

[jdbosser]

Describe the bug
Size of circles are not "respecting" the cartesian grid. Drawing the same circle in two different grids yield a circle that is equally large in the two images, and is not scaled to match the coordinate system difference. Is this intentional?

To Reproduce
I took the code from the snowflake example, and added a circle. Change the line
.build_cartesian_2d(-2.0..2.0, -1.5..1.5)?;
to
.build_cartesian_2d(-4.0..4.0, -3.0..3.0)?;
and then re-export and compare the figures.

Including the snowflake code with a circle in it for completeness.

use plotters::prelude::*;

fn snowflake_iter(points: &[(f64, f64)]) -> Vec<(f64, f64)> {
    let mut ret = vec![];
    for i in 0..points.len() {
        let (start, end) = (points[i], points[(i + 1) % points.len()]);
        let t = ((end.0 - start.0) / 3.0, (end.1 - start.1) / 3.0);
        let s = (
            t.0 * 0.5 - t.1 * (0.75f64).sqrt(),
            t.1 * 0.5 + (0.75f64).sqrt() * t.0,
        );
        ret.push(start);
        ret.push((start.0 + t.0, start.1 + t.1));
        ret.push((start.0 + t.0 + s.0, start.1 + t.1 + s.1));
        ret.push((start.0 + t.0 * 2.0, start.1 + t.1 * 2.0));
    }
    ret
}

const OUT_FILE_NAME: &str = "snowflake.png";
fn main() -> Result<(), Box<dyn std::error::Error>> {
    let root = BitMapBackend::new(OUT_FILE_NAME, (1024, 768)).into_drawing_area();

    root.fill(&WHITE)?;

    let mut chart = ChartBuilder::on(&root)
        .caption("Koch's Snowflake", ("sans-serif", 50))
        .build_cartesian_2d(-2.0..2.0, -1.5..1.5)?;

    let mut snowflake_vertices = {
        let mut current: Vec<(f64, f64)> = vec![
            (0.0, 1.0),
            ((3.0f64).sqrt() / 2.0, -0.5),
            (-(3.0f64).sqrt() / 2.0, -0.5),
        ];
        for _ in 0..6 {
            current = snowflake_iter(&current[..]);
        }
        current
    };

    chart.draw_series(std::iter::once(Polygon::new(
        snowflake_vertices.clone(),
        RED.mix(0.2),
    )))?;
    snowflake_vertices.push(snowflake_vertices[0]);
    chart.draw_series(std::iter::once(PathElement::new(snowflake_vertices, RED)))?;

    chart.draw_series(std::iter::once(Circle::new((0.0, 0.0), 20.0, ShapeStyle {
        color: GREEN.into(),
        filled: true, 
        stroke_width: 2
    })))?;

    // To avoid the IO failure being ignored silently, we manually call the present function
    root.present().expect("Unable to write result to file, please make sure 'plotters-doc-data' dir exists under current dir");
    println!("Result has been saved to {}", OUT_FILE_NAME);
    Ok(())
}
#[test]
fn entry_point() {
    main().unwrap()
}

This produces these two images. One can see that PathElement respects the coordinate system as the star changes size in the image, but the Circle does not.

Using .build_cartesian_2d(-2.0..2.0, -1.5..1.5)?;:

Using .build_cartesian_2d(-4.0..4.0, -3.0..3.0)?;:

Version Information
Installed using cargo add plotters. I have version 0.3.7.