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

### 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

Start with the minimal example and inspect vg.Path.Arc together with the canvas Stroke call; the issue provides the exact reproduction and expected geometry. Verify the fix by generating the PNG and confirming that the upper semicircle appears alongside the axes and title.

Scritto dal modello di indicizzazione a partire dal testo della issue.