[BUG] Drawing off by a few px

[jangernert]

Describe the bug
At least with the CanvasBackend the drawing of the axis seems to be off a little. Some lines of the axis don't quite connect. Some overlap which looks bad with the default opacity of the line.

It seems like the thickness of the line is not taken into consideration when drawing.

Overlap:

Overlap and end of X-Axis a little too short:

Axis not quite connecting right at 0:

Top of Y-Axis not going all the way to the top

To Reproduce

let backend = CanvasBackend::new("canvas").unwrap();
let root = backend.into_drawing_area();

root.fill(&WHITE).unwrap();

let mut chart = ChartBuilder::on(&root)
    .set_left_and_bottom_label_area_size(30)
    .margin(10)
    .build_ranged(0.0..2.0, 0.0..3.0)
    .unwrap();

chart
    .configure_mesh()
    .x_labels(3)
    .y_labels(2)
    .draw()
    .unwrap();

let data = (0..=20)
    .map(|x| x as f64 / 10.0)
    .map(|x| (x, x.powf(2.0) + 1.0));

chart.draw_series(
    AreaSeries::new(
        data.clone(),
        0.0,
        &RED.mix(0.2),
    )
    .border_style(&RED),
).unwrap();

chart.draw_series(
    data
        .map(|(x, y)| Circle::new((x, y), 3, BLUE.filled())),
).unwrap();

root.present().unwrap();

Version Information

plotters = "0.2"

[Pascal-So]

It seems like this is related to #128, specifically the fact that plotters tries to draw the lines at integer coordinates but when the backend supports float coordinates then we have to add 0.5 to the coordinates, otherwise we're drawing "between" pixels. See my PR in plotters-canvas for an attempt at fixing this.