Fix plotting with 1 channel

Author: F33RNI

GraphPlot.py

@@ -112,7 +112,7 @@ def plot_data(self, frequencies, levels, normalize: bool,
         :param normalize_ref: normalize reference data before applying it
         :return:
         """
-        if levels is not None and len(levels) > 1:
+        if levels is not None and len(levels) > 0:
             # Coppy levels array
             levels_copy = levels.copy()