Don,
In your config.txt for wxgraphic.php, you do have $degree_units = '°F';
which is in ISO-8859-1.
You'll likely need to add to your php.ini in your document root default_charset = "ISO-8859-1"
As of PHP 5.6+, PHP changed from internal coding of ISO-8859-1 to UTF-8. The GD routines (which draw the text on the image) use the internal PHP coding for the characters, so the ISO-8859-1 spec for your $degree_units is written as UTF-8 (unless the default_charset is set to ISO-8859-1)
Yes, the default charset for all the Saratoga templates is ISO, not UTF-8 (unless forced by a per-page spec to be UTF-8).
Best regards,
Ken