From fa1be1e5bf50c9da27cc9473b04f2399319dc324 Mon Sep 17 00:00:00 2001
From: Martin Chase <outofculture@gmail.com>
Date: Wed, 5 May 2021 17:08:04 -0700
Subject: [PATCH] show and hide properly for maxTraces; +test (#1764)

---
 pyqtgraph/graphicsItems/PlotItem/PlotItem.py  | 25 +++++++++----------
 .../PlotItem/tests/test_PlotItem.py           | 17 +++++++++++++
 2 files changed, 29 insertions(+), 13 deletions(-)

diff --git a/pyqtgraph/graphicsItems/PlotItem/PlotItem.py b/pyqtgraph/graphicsItems/PlotItem/PlotItem.py
index 8da9ba39..c7d47631 100644
--- a/pyqtgraph/graphicsItems/PlotItem/PlotItem.py
+++ b/pyqtgraph/graphicsItems/PlotItem/PlotItem.py
@@ -1003,20 +1003,19 @@ class PlotItem(GraphicsWidget):
         return self.ctrl.clipToViewCheck.isChecked()
 
     def updateDecimation(self):
-        if self.ctrl.maxTracesCheck.isChecked():
-            numCurves = self.ctrl.maxTracesSpin.value()
+        if not self.ctrl.maxTracesCheck.isChecked():
+            numCurves = len(self.curves)
         else:
-            numCurves = -1
-            
-        curves = self.curves[:]
-        split = len(curves) - numCurves
-        for curve in curves[split:]:
-            if numCurves != -1:
-                if self.ctrl.forgetTracesCheck.isChecked():
-                    curve.clear()
-                    self.removeItem(curve)
-                else:
-                    curve.hide()        
+            numCurves = self.ctrl.maxTracesSpin.value()
+
+        for i, curve in enumerate(self.curves):
+            if i < numCurves:
+                curve.show()
+            elif self.ctrl.forgetTracesCheck.isChecked():
+                curve.clear()
+                self.removeItem(curve)
+            else:
+                curve.hide()
       
     def updateAlpha(self, *args):
         (alpha, auto) = self.alphaState()
diff --git a/pyqtgraph/graphicsItems/PlotItem/tests/test_PlotItem.py b/pyqtgraph/graphicsItems/PlotItem/tests/test_PlotItem.py
index 83d2531d..9c464ffd 100644
--- a/pyqtgraph/graphicsItems/PlotItem/tests/test_PlotItem.py
+++ b/pyqtgraph/graphicsItems/PlotItem/tests/test_PlotItem.py
@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
+import numpy as np
 import pytest
+
 import pyqtgraph as pg
 
 app = pg.mkQApp()
@@ -23,6 +25,21 @@ def test_PlotItem_shared_axis_items(orientation):
         pi2.setAxisItems({orientation: ax1})
 
 
+def test_PlotItem_maxTraces():
+    item = pg.PlotItem()
+
+    curve1 = pg.PlotDataItem(np.random.normal(size=10))
+    item.addItem(curve1)
+    assert curve1.isVisible(), f"{curve1} should be visible"
+
+    item.ctrl.maxTracesCheck.setChecked(True)
+    item.ctrl.maxTracesSpin.setValue(0)
+    assert not curve1.isVisible(), f"{curve1} should not be visible"
+
+    item.ctrl.maxTracesCheck.setChecked(False)
+    assert curve1.isVisible(), f"{curve1} should be visible"
+
+
 def test_plotitem_menu_initialize():
     """Test the menu initialization of the plotitem"""
     item = pg.PlotItem()