Eventually I put a big comment at the top that to understand the code you needed to draw the picture, and named my variables x and y. About a page of code later, I was done. And when I has to tinker with it a couple of years later my first reaction was to wonder WTF I was thinking, wince in memory of how I had struggled with it, then I drew the picture, and I was amazed at how easy it was.
The rule is not that you need long or short variables. You need meaningful ones. A short variable name is inherently ambiguous, which can lead to confusion and mistakes. Thinking through anything with a long-variable name abuses your working memory, limiting how complex your thoughts can be.
It is a trade-off. Use the right one for your code. For me, index variables are short, and so are any variables that refer to math concepts that I understand well. Otherwise I use (concise if possible) descriptions without any abbreviation, separated by underscores.