We were looking at this graph in the Members Academy today, and a few students wrote "250 thousand people" (Number + Word format).
I felt a little uncomfortable about it but I didn't correct it. I didn't think it was a serious error.
However, one of my students sent me some feedback he'd had from a previous IELTS tutor which said "We don't write numbers like this".
(This is what I love about the Members Academy !)
So I did what I always tell my students to do (and what I should have done in the first place).
I went to my favourite research place (Ludwig.guru) and found hundreds of examples of reliable, academic texts which used the 'Number + Word' format for large numbers.
So if someone tells you to do something or NOT to do something, ALWAYS look for evidence first (ask me how to do this if you're not sure).
There are other ways of avoiding large numbers in a graph like this.
After all, it's a test of English not of numbers, so the more words you use, the better.