If they have an Android device or you have a spare one, you can use Andie graph. This is what I used to get me through college since I was too cheap to buy a new graphing calc, hah. I was in the Play Store when I first downloaded it, but it seems to have been removed and I had to re-download it from the link given to get it back on my new phones. Else it seems to have been since replaced with AlmostTI, which costs $5.
If you can't find the ROM files needed for the app to work, PM me and I'll send you a zip with all the TI ROMs I have for it.
Thanks for the tip. Sibling is equipped with iphone, not even sure which one, 8 maybe. If it were for my self I'd be down to try it but I can imagine there's a practicality to having a purpose built device for the maths. Also button/menu layout for ease of use in a classroom with others.