This isn't a grammar question. Your looking for the best way to present infomation without it being confusing. This is more of a visual problem. I'd separate the temperatures somehow. Maybe:
(-x) - (-y)
or replace the dash with a different symbol or word that doesn't so closely resemble the - sign.
-x to -y
-x - -y
Or a visibly longer dash? If you don't create separation somehow, you will end up with a problem, especially if you get into positive numbers for the second digit.
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