diff --git a/swash/processing/post.py b/swash/processing/post.py
index 4ad36b1..ec4977c 100644
--- a/swash/processing/post.py
+++ b/swash/processing/post.py
@@ -7,6 +7,7 @@ import matplotlib.pyplot as plt
 import numpy as np
 import pandas as pd
 import scipy.signal as sgl
+import scipy.fft as fft
 
 from .read_swash import *
 
@@ -68,9 +69,18 @@ ax_vel.axvline(t0, c="k", alpha=0.2)
 fig.tight_layout()
 
 fig_r, ax_r = plt.subplots()
+ax_fft = ax_r.twinx()
 
-ax_r.plot(1 / phi_eta[0, 1:], R[1:], marker="+", lw=1)
-ax_r.set(xlim=(1, 30), ylim=(0, 1), xlabel="t (s)", ylabel="R")
+ax_fft.plot(
+    fft.rfftfreq(eta.size, dt),
+    np.abs(fft.rfft(eta)),
+    lw=1,
+    c="k",
+    alpha=0.2,
+)
+ax_r.plot(phi_eta[0], R, marker="+")
+ax_r.set(xlim=(0, 0.3), ylim=(0, 1), xlabel="f (Hz)", ylabel="R")
+ax_fft.set(ylim=0, ylabel="FFT")
 ax_r.grid()
 
 fig_x, ax_x = plt.subplots()
@@ -88,8 +98,8 @@ out = pathlib.Path(config.get("post", "out")).joinpath(f"t{t0}x{x0}")
 log.info(f"Saving plots in '{out}'")
 out.mkdir(parents=True, exist_ok=True)
 
-fig.savefig(out.joinpath(f"t.png"))
-fig_r.savefig(out.joinpath(f"R.png"))
-fig_x.savefig(out.joinpath(f"x.png"))
+fig.savefig(out.joinpath("t.png"))
+fig_r.savefig(out.joinpath("R.png"))
+fig_x.savefig(out.joinpath("x.png"))
 
 log.info("Finished post-processing")