How to get a square axis?

[lalala-233]

I want to draw a square axis, but the axis will be stretched. What should I do?

let root = BitMapBackend::new("output.png", (768, 1080)).into_drawing_area();
root.fill(&WHITE)?;
let mut chart = ChartBuilder::on(&root)
        .caption("Linear Regression", ("sans-serif", 50).into_font())
        .margin(10)
        .x_label_area_size(30)
        .y_label_area_size(30)
        .build_cartesian_2d(-0.15..0.15, -0.15..0.15)?;