Maybe 1 volt in addition to your input voltage in a summing amplifier based on op amp (741,lm324,op07,ne5532, or any else hanging on your bench) will be enough too ( 2 volt also).

0v output of op amp will mean -1v
1v output of op amp will mean 0v
2v output of op amp will mean +1v

will be easy to convert in a readable VAR with 2 IF THEN ELSE or 1 SELECT CASE with 2 branchs.

regards