Line graph doesn not work. No common join columns detected error

I have three columns(say A,B,C). I tried to keep column A in x-axis and column B in Y-axis. the line graph works fine. But as soon as I add column C as legend it gives me error --"Query(131, 13) no common join columns detected. The join "NATURAL INNERJOIN" requires atleast one common join column". I tried doing the same in excel and it works. I have a feeling I am missing something trivial. Did anybody face this problem before ?