Rust中使用Plotters添加PointSeries绘图报错求助

尝试用Plotters库生成同时包含折线和点系列的图表，折线系列正常，但点系列抛出类型推断错误，简化代码及错误信息如下：

问题代码 #

fn main() {
    let points: Vec<(f64, f64)> = vec![(2.0, 2.0), (3.0, 3.0 ), (4.0, 4.0), (8.0, 8.0)];
    generate_plot(&points);
}

fn generate_plot(points: &Vec<(f64, f64)>) -> Result<(), Box<dyn std::error::Error>> {
    let root = BitMapBackend::new("C:\\Users\\ppp\\Documents\\Rust\\tpy\\Data\\test.png", (800, 800)).into_drawing_area();
    root.fill(&WHITE)?;
    let mut chart = ChartBuilder::on(&root)
       .build_cartesian_2d(0f64..10f64, 0f64..10f64)?;
    chart.configure_mesh().draw()?;
    chart.draw_series(PointSeries::new(points.clone(), 5, &BLUE))?;  //此处报错
    root.present()?;
    Ok(())
}

错误信息 #

error[E0282]: type annotations needed
--> src\main.rs:29:11
| 29 | chart.draw_series(PointSeries::new(points.clone(), 5, &BLUE))?;
| ^^^^^^^^^^^ cannot infer type of the type parameter B declared on the associated function draw_series

error[E0283]: type annotations needed
--> src\main.rs:29:23
| 29 | chart.draw_series(PointSeries::new(points.clone(), 5, &BLUE))?;
| ^^^^^^^^^^^^^^^^ cannot infer type of the type parameter E declared on the struct PointSeries
| = note: multiple impls satisfying _: PointElement<(f64, f64), i32> found in the plotters crate:

• impl<Coord, Size> PointElement<Coord, Size> for Circle<Coord, Size> where Size: SizeDesc;
• impl<Coord, Size> PointElement<Coord, Size> for Pixel where Size: SizeDesc;
• impl<Coord, Size> PointElement<Coord, Size> for TriangleMarker<Coord, Size> where Size: SizeDesc;
• impl<Coord, Size> PointElement<Coord, Size> for plotters::element::Cross<Coord, Size> where Size: SizeDesc;

解决方案 #

报错原因是编译器无法自动推断PointSeries使用的点元素类型（Plotters提供了Circle、Pixel、TriangleMarker等多种点元素实现），与点大小参数无关。只需显式指定点元素类型即可解决，以下是两种可行修改方式：

方式1：使用element方法指定点元素 #

修改draw_series调用行，通过element::<Circle<_>>()明确使用圆形点：

chart.draw_series(PointSeries::new(points.clone(), 5, &BLUE).element::<Circle<_>>())?;

方式2：显式标注PointSeries的类型参数 #

直接在PointSeries::new处指定类型参数，以Circle为例：

chart.draw_series(PointSeries::<_, _, _, Circle<_>, _>::new(points.clone(), 5, &BLUE))?;

完整修正代码 #

use plotters::prelude::*;

fn main() {
    let points: Vec<(f64, f64)> = vec![(2.0, 2.0), (3.0, 3.0 ), (4.0, 4.0), (8.0, 8.0)];
    if let Err(e) = generate_plot(&points) {
        eprintln!("Error: {}", e);
    }
}

fn generate_plot(points: &Vec<(f64, f64)>) -> Result<(), Box<dyn std::error::Error>> {
    let root = BitMapBackend::new("C:\\Users\\ppp\\Documents\\Rust\\tpy\\Data\\test.png", (800, 800)).into_drawing_area();
    root.fill(&WHITE)?;
    let mut chart = ChartBuilder::on(&root)
       .build_cartesian_2d(0f64..10f64, 0f64..10f64)?;
    chart.configure_mesh().draw()?;
    // 使用方式1的修正代码
    chart.draw_series(PointSeries::new(points.clone(), 5, &BLUE).element::<Circle<_>>())?;
    root.present()?;
    Ok(())
}

内容的提问来源于stack exchange，提问作者PMG