Why do I have two waveforms in my plot while I should have only one?

I import a matrix from CST and plot 2 columns and get two different waveforms. Why is this happening since x and y are 360x1 columns?

[Mathieu NOE]

even if you data is 1D this could be a concatenation of two measurements , so you would also get 2 lines

make sure you have unique x and y

or maybe you already made a first plot with hold on and you still have this first trace