Since you started with a line, the iso-surface based on this line will be a point. In order to achieve your desired result, you will need to start with a vertical plane, The iso-suface would be a curve. Once you have the curve, you should be able to plot the y'coordinate of this curve along the length of your domain. If you are looking for the result as a function of space at a given time-step, you can do this as a post-processing step. Havign said that, I'd strongly recommend that you create these post-processing instances at the beginning of your simulation and write the results periodically at various time steps. This will give you a progression of what these waves look like in time.
Hope this helps answer your question.