diff --git a/src/f4enix/output/rssa/rssa.py b/src/f4enix/output/rssa/rssa.py
index 7954ec8..b6415e7 100644
--- a/src/f4enix/output/rssa/rssa.py
+++ b/src/f4enix/output/rssa/rssa.py
@@ -1,6 +1,4 @@
 """This module is related to the parsing of D1S-UNED meshinfo files."""
-
-from collections.abc import Sequence
 from pathlib import Path
 
 import polars as pl
@@ -140,9 +138,6 @@ def histories(self) -> pl.Series:
         """Returns the history numbers of the tracks."""
         return self.tracks["a"]
 
-    def get_energy_spectra(self, energy_bins: Sequence[float]) -> pl.DataFrame:
-        raise NotImplementedError()
-
     def plot_plane(self) -> RSSAPlot:
         """Returns an instance of RSSAPlot to plot data assuming an XY plane."""
         return RSSAPlot(self.tracks, self.parameters)
diff --git a/src/f4enix/output/rssa/rssa_plotting.py b/src/f4enix/output/rssa/rssa_plotting.py
index e658f86..dd83cce 100644
--- a/src/f4enix/output/rssa/rssa_plotting.py
+++ b/src/f4enix/output/rssa/rssa_plotting.py
@@ -80,9 +80,14 @@ def calculate_perimeter_positions(self) -> None:
         the X and Y coordinates and calculates the position as a perimeter coordinate.
         The perimeter position is calculated as theta * r, where theta is the angle in
         radians and r is the radius of each track.
+
+        The theta is calculated using the arctan2 function, which gives the angle
+        between the positive X-axis and the point (x, y). In that calculation, the X
+        coordinate is multiplied by -1 to "flip" the plot, this way it looks as a
+        cylinder observed from the inside.
         """
         radius = (pl.col("x").pow(2) + pl.col("y").pow(2)).sqrt()
-        thetas = pl.arctan2(pl.col("y"), pl.col("x"))
+        thetas = pl.arctan2(pl.col("y"), -1 * pl.col("x"))
         perimeter_pos = (thetas * radius).alias("perimeter_pos")
         self.tracks = self.tracks.with_columns(perimeter_pos)