The Velleman HPS140 digital oscilloscope at around £100 ($150) is a great little instrument but suffers from only having edge-scales rather than a full graticule. Since the device is good enough for rough 'n ready measurements of fairly high standard, and given that my old eyes can struggle a bit with tiny things I turned to the interweb for an answer.
Found just one on the Velleman site where a chap said he'd drawn marker-pen lines on a slice of screensaver. I wanted just a little bit better than this and in spite of endless searching came to the conclusion that I was going to have to do it myself.
More searching found nothing very useful in the way of grid-drawing software so I turned to Microsoft Publisher (running in Parallels on a Mac - don't worry if that means nothing to you) and half by calculation and half by empirical 'bit-bigger-that-way' twiddling ended up with the attached file.
I printed half a dozen copies on a single sheet of acetate because the acetate was patchy and I also knew it would take several attempts to cut it to fit. It did!
I'm NOT a perfectionist and good enough was near enough for me - you may want to do better...
Vertical 0.7mm for each of 24 on-screen dots = 16.9mm@ 3 divs per 'dash'
Horizontal 0.7mm for each of 58 on-screen dots = 40.9mm @ 5 divs per dash
Offset from origin is arbitrary and done by eye with scissors later.
Laser printed - the actetate curled badly (old, damp?) but fortunately I'd done many copies per sheet; then roughly cut out.
I tried using the ink-jet with a pale blue grid but it smudged too easily.
Make sure you put the printed side facing inwards.
With the final version I left the bottom-most line un-printed since the alignment discrepancy didn't show so much that way.
(like I said: 'near enough is...')
Now to cutting. I settled on four attempts before reminding myself that 'good e...' - you know the rest!
Used a scalpel tip to roughly mark the corners then trimmed down bit by bit until it (near enough) fitted.
Final step is making it stay in place. At the moment it's a push-fit and seems to hold fairly well; I don't want to spray glue it because the device's front screen seems pretty flimsy (and prone to scratches) as it is.
The loose fit lets me jiggle it around when I need better alignment but is only half an answer (and a whole excuse).
So it's over to the massed ranks of Instructable geniuses (genii?) to come up with an idea for final fixing.
Over to you!
(oh, and as soon as I can work out where to put the Publisher file, you can have a copy! Meanwhile email me at firstname.lastname@example.org if you want a copy)