Merge pull request #2443 from SasView/1329-meaningless-error-values-o…

…n-theory-and-normalised-residuals

Add +/- 1 and 3 sigma lines on residual plots

src/sas/qtgui/Plotting/Plotter.py

@@ -227,6 +227,12 @@ def plot(self, data=None, color=None, marker=None, hide_error=False, transform=T
         if data.show_yzero:
             ax.axhline(color='black', linewidth=1)
 
+        # Display +/- 3 sigma and +/- 1 sigma lines for residual plots
+        if data.plot_role == data.ROLE_RESIDUAL:
+            ax.axhline(y=3, color='red', linestyle='-')
+            ax.axhline(y=-3, color='red', linestyle='-')
+            ax.axhline(y=1, color='gray', linestyle='--')
+            ax.axhline(y=-1, color='gray', linestyle='--')
         # Update the list of data sets (plots) in chart
         self.plot_dict[data.name] = data