Position difference between locations of individual FP lines and a smooth wavelength solution. The smooth wavelength solution was obtained in a nonparametric way by applying a kernel smoothing filter (basically a low-pass) to the FP line positions. Individual points are connected with lines to better visualize the quasi-periodic behavior. The left panel shows a histogram of these smoothing residuals with indications for the 16th, 50th, and 84th percentile of the distribution. The mean formal position error of the lines is visualized by a red bar.