diff --git a/ohmpi/hardware_system.py b/ohmpi/hardware_system.py
index 38fc63fa4bcabe52ac34fe0b3cf2605a77575eaf..b20587487e633e5919fb27a18a533105f4dc6197 100644
--- a/ohmpi/hardware_system.py
+++ b/ohmpi/hardware_system.py
@@ -324,7 +324,7 @@ class OhmPiHardware:
         ax[2].plot(self.readings[:, 0], self.readings[:, 2], '-g', marker='.', label='polarity')
         ax[2].set_ylabel('polarity [-]')
         v = self.readings[:, 2] != 0
-        ax[3].plot(self.readings[:, 0], (self.readings[v, 2] * (self.readings[v, 4] - self.sp)) / self.readings[v, 3],
+        ax[3].plot(self.readings[v, 0], (self.readings[v, 2] * (self.readings[v, 4] - self.sp)) / self.readings[v, 3],
                    '-m', marker='.', label='R [ohm]')
         ax[3].set_ylabel('R [ohm]')
         fig.legend()