From f1de464c460e9cbdb0987a4cf85b76930e88ad18 Mon Sep 17 00:00:00 2001
From: Luke Campagnola <ausqualius@yahoo.com>
Date: Wed, 4 Oct 2017 08:30:38 -0700
Subject: [PATCH] Preserve levels when switching between mono and rgba modes

---
 pyqtgraph/graphicsItems/HistogramLUTItem.py | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/pyqtgraph/graphicsItems/HistogramLUTItem.py b/pyqtgraph/graphicsItems/HistogramLUTItem.py
index 68448c11..019fa3a7 100644
--- a/pyqtgraph/graphicsItems/HistogramLUTItem.py
+++ b/pyqtgraph/graphicsItems/HistogramLUTItem.py
@@ -288,9 +288,21 @@ class HistogramLUTItem(GraphicsWidget):
         Options are 'mono' or 'rgba'.
         """
         assert mode in ('mono', 'rgba')
+        
+        oldLevels = self.getLevels()
+        
         self.levelMode = mode
         self._showRegions()
         self.imageChanged()
+        
+        # do our best to preserve old levels
+        if mode == 'mono':
+            levels = np.array(oldLevels).mean(axis=0)
+            self.setLevels(*levels)
+        else:
+            levels = [oldLevels] * 4
+            self.setLevels(rgba=levels)
+        
         self.update()
 
     def _showRegions(self):