Try changing your Signals calc from Float([Wait Time]) to Sum(Float([Wait Time])), therefore the total of the wait time will be included in the If statement.
I think you're getting the second axis because you're looking at AVERAGES, and some of your results that make your daily average are reasonable, and some are not.
If you change this in your expression:
if avg(float([Wait Time])) >= 20 then "Wait GTE 20 min"
else "Reasonable Wait"
You may get what you're looking for.