The canvas.Line object draws a line from the Position1 (default is top, left) to Position2 (default is bottom, right). You specify its colour and can vary the stroke width which otherwise defaults to 1.

A line position can be manipulated using the Position1 or Position2 fields or by using the Move() and Resize() functions. For example a 0 width area will show a vertical line whereas 0 height would be horizontal.

package main

import (
	"image/color"

	"fyne.io/fyne/v2"
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/canvas"
)

func main() {
	myApp := app.New()
	w := myApp.NewWindow("Line")

	line := canvas.NewLine(color.White)
	line.StrokeWidth = 5
	w.SetContent(line)

	w.Resize(fyne.NewSize(100, 100))
	w.ShowAndRun()
}

Lines are typically used in a custom layout or controlled manually. Unlike text they have no natural (minimum) size but can be used to great effect in complex layouts.