Changed region check to use exception

scripts/Calibration/tofpd/diagnostics.py

@@ -501,11 +501,9 @@ def plot_peakd(wksp: Union[str, Workspace2D], peak_positions, plot_regions=True,
             dend = single.yIndexOfX(int(x.searchsorted(drange[1])))
             print("Specified ending detector range was not valid, adjusted to detID={}".format(dend))
 
-        if dstart <= dend:
-            cut_id = (dstart, dend)
-        else:
-            # Switch range if the start point is larger
-            cut_id = (dend, dstart)
+        if dstart >= dend:
+            raise RuntimeError("Detector start region ({}) must be less than the end region ({})".format(dstart, dend))
+        cut_id = (dstart, dend)
 
         # skip if y was entirely nans or detector slice yields empty region
         if len(y_val) == 0 or len(y_val[cut_id[0]:cut_id[1]]) == 0: