Use the azimuthal lineout range in WPPF difference

This gives a better feel for how much error there is, on a first
glance.

The difference plot is always *at least* the same as the lineout
range, but the top and bottom are stretched to contain the whole
difference plot.

Fixes a subissue in #1926

Signed-off-by: Patrick Avery <[email protected]>

Author: psavery

hexrdgui/image_canvas.py

@@ -1823,6 +1823,13 @@ def update_wppf_plot(self):
 
         last_lineout = HexrdConfig().last_unscaled_azimuthal_integral_data
         diff_axis = self.wppf_difference_axis
+
+        # Rescale.
+        # This actually ignores the scatter plot data when rescaling,
+        # which is fine. We will stay zoomed in on the line.
+        axis.relim()
+        axis.autoscale_view(scalex=False)
+
         if diff_axis and wppf_data and last_lineout:
             style = {
                 'c': '#000000',
@@ -1841,15 +1848,15 @@ def update_wppf_plot(self):
             diff_axis.relim()
             diff_axis.autoscale_view(scalex=False)
 
+            # When the difference plot resets, always set it to be
+            # initially the same y range as the azimuthal.
+            new_ymin = min(diff_axis.get_ylim()[0], axis.get_ylim()[0])
+            new_ymax = max(diff_axis.get_ylim()[1], axis.get_ylim()[1])
+            diff_axis.set_ylim((new_ymin, new_ymax))
+
         # Update the difference label even if there's no data
         self.update_wppf_difference_labels()
 
-        # Rescale.
-        # This actually ignores the scatter plot data when rescaling,
-        # which is fine. We will stay zoomed in on the line.
-        axis.relim()
-        axis.autoscale_view(scalex=False)
-
     def update_wppf_difference_labels(self):
         axis = self.wppf_difference_axis
         if not axis: