Don't crash when the variable is missing

interpret_qc.py

@@ -53,6 +53,8 @@ def save_plot(filename):
 
 def plot_variable_qc(frame, loc, filename=None):
     qc_loc = 'qc_' + loc
+    if qc_loc not in frame.columns:
+        return
     if sum(frame[qc_loc]) > 0:
         fig = plt.figure(figsize=(50,5))
         plot_outliers(frame, frame[qc_loc] == 0, loc)