plot/vg: Unable to add or visualize an Arc from vg.Path

[JerPatton]

What are you trying to do?

I am trying to draw arcs on a plot and here as an example I specifically try to display an arc with center (0,0) and radius of 1 from the angle 0 to pi and save that to a .png file.

What did you do?

Here is a minimal working example of the code I used:

package main

import (
	"math"

	"gonum.org/v1/plot"
	"gonum.org/v1/plot/vg"
	"gonum.org/v1/plot/vg/draw"
	"gonum.org/v1/plot/vg/vgimg"
)

func main() {
	p := plot.New()
	p.Title.Text = "Arc Plot Example"
	p.X.Label.Text = "X"
	p.Y.Label.Text = "Y"

        // Create Canvas
	cnvs := vgimg.PngCanvas{
		Canvas: vgimg.New(5*vg.Centimeter, 5*vg.Centimeter),
	}

	// Draw arc
	centerX := 0.0      // X-coordinate of the center of the circle
	centerY := 0.0      // Y-coordinate of the center of the circle
	radius := 1.0       // Radius of the circle
	startAngle := 0.0   // Start angle of the arc (in radians)
	endAngle := math.Pi // End angle of the arc (in radians)

	path := vg.Path{}
	path.Arc(vg.Point{X: vg.Length(centerX), Y: vg.Length(centerY)}, vg.Length(radius), startAngle, endAngle) // Add the arc to the path
	cnvs.Stroke(path)

        // Add the canvas and the Arc to the plot
	p.Draw(draw.New(cnvs))

	// Save plot and also try to save canvas to file directly
	
	// Save the plot to a PNG file.
	if err := p.Save(5*vg.Centimeter, 5*vg.Centimeter, "figures/arc1.png"); err != nil {
		panic(err)
	}

	// Save the canvas directly to a PNG file.
	w, err := os.Create("figures/arc2.png")
	if err != nil {
		panic(err)
	}
	defer w.Close()
	if _, err := cnvs.WriteTo(w); err != nil {
		panic(err)
	}

	// pic.ShowImage(cnvs.Image())
}

I tried to use the go playground and pic.ShowImage(img) as suggested but it won't compile. Still, find the link below if it can work for you...
https://go.dev/play/p/KMPRsOUcO-0

What did you expect to happen?

It should draw the arc (line) as described above, i.e. we should see half a circle spanning from -1 to 1 with a radius of 1 on the side of the positive y-axis values.

What actually happened?

Nothing is drawn to the image except the axes and title.

What version of Go and Gonum/plot are you using?

go 1.20
gonum.org/v1/plot v0.13.0

[kortschak]

Try setting the line style. See https://pkg.go.dev/gonum.org/v1/plot/vg/draw#Canvas.SetLineStyle.

[JerPatton]

Unfortunately the arc is still not there.

Am I missing something in the process or is it indeed a bug?
Could you maybe point me towards a working example using an Arc or a Path? So that I can test further.

Thanks in advance

[kortschak]

The rings package makes extensive use of arcs, so maybe try looking there (example).