Here's a different suggestion: I wouldn't place a hyphen before the word "plane" but, instead, an unbreakable space. Second, I'd insert an (unbreakable) en-dash between $x$ and $y$:
The en-dash is, typographically speaking, the most frequently used "connector" symbol, as in "pages 40--52", "Einstein--Bose condensates", "categories A--Z", etc. The notation implies, so to say, that we're talking about the plane formed by (linear combinations of) points on the $x$ and $y$ axes.
I'd note, separately, that I wouldn't foresee much of a chance for confusion between $x$--$y$ and $x-y$, because TeX inserts whitespace (of width
\medmuskip) around binary operators (such as the minus sign):
Note also that the endash is thinner than the math-minus sign.