Simple fill pattern in svg : diagonal hatching

I did not find anything for diagonal hatching on the internet either, so I'll share my solution here:

<pattern id="diagonalHatch" patternUnits="userSpaceOnUse" width="4" height="4">
  <path d="M-1,1 l2,-2
           M0,4 l4,-4
           M3,5 l2,-2" 
        style="stroke:black; stroke-width:1" />
</pattern>

(note the lower case "l" in the path expression)

The above creates a hatch with diagonal lines from the lower left to the upper right that are 4 pixels apart. Besides the diagonal line (M0,4 l4,-4) you also have to stroke the upper left and the lower right edges of the pattern area, since the line will otherwise be "constricted" due to clipping where it intersects the edges of the square.

To fill a rectangle with this pattern, do:

<rect x="0" y="0" width="100%" height="100%" fill="url(#diagonalHatch)"/>

Use the patternTransform attribute to rotate a vertical (or horizontal) line segment. This method tiles seamlessly and uses the simplest possible path. The pattern width attribute controls how close parallel hatches are.

<pattern id="diagonalHatch" width="10" height="10" patternTransform="rotate(45 0 0)" patternUnits="userSpaceOnUse">
  <line x1="0" y1="0" x2="0" y2="10" style="stroke:black; stroke-width:1" />
</pattern>

You may be able to create, what you want using a <pattern> tag.

As a starting point you might take this example of the respective MDN docu:

    <?xml version="1.0"?>
    <svg width="120" height="120" viewBox="0 0 120 120"
         xmlns="http://www.w3.org/2000/svg" version="1.1"
         xmlns:xlink="http://www.w3.org/1999/xlink">
     
        <defs>
            <pattern id="Triangle"
                     width="10" height="10"
                     patternUnits="userSpaceOnUse">
                <polygon points="5,0 10,10 0,10"/>
            </pattern>
        </defs>
     
        <circle cx="60" cy="60" r="50"
                fill="url(#Triangle)"/>
    </svg>