I did a plot where I have two key columns: One numerical and one string. When trying to add an x refline on 120, I got must have 2 x values, had 1 values: 120. I'm guessing it's confused because there are two x columns. Out of curiosity I also tried 120 greedy, greedy 120 and 120,greedy and it failed on all of these.

@midichef