Change of Category (X) axis labels

Hi, I would like to ask you for help with change of (X) axis labels in my graph. I have those source data:

ot1 = (question n. 1)
a 0
b 0
c 4
d 0

created graph you can see as a picture.

Now, I would like to change 'a', 'b', 'c' and 'd' to some text (answers) like 'Yes', 'No', 'Not sure' .... etc, but source data must stay as you see. The problem is that answers are not same for all questions...

regards
Patrik