diff --git a/rtgui/mydiagonalcurve.cc b/rtgui/mydiagonalcurve.cc
index b0fe4d66e..193c7dbef 100644
--- a/rtgui/mydiagonalcurve.cc
+++ b/rtgui/mydiagonalcurve.cc
@@ -138,7 +138,7 @@ void MyDiagonalCurve::draw (int handle) {
         return;
 
     // re-calculate curve if dimensions changed
-    if (prevGraphW != graphW || prevGraphH != graphH || int(point.size()) != graphW/4)
+    if (prevGraphW != graphW || prevGraphH != graphH || int(point.size()) != graphW-2)
         interpolate ();
 
     Gtk::StateType state = !is_sensitive() ? Gtk::STATE_INSENSITIVE : Gtk::STATE_NORMAL;
diff --git a/rtgui/myflatcurve.cc b/rtgui/myflatcurve.cc
index 2e04f6d46..9e347620a 100644
--- a/rtgui/myflatcurve.cc
+++ b/rtgui/myflatcurve.cc
@@ -96,7 +96,7 @@ void MyFlatCurve::draw () {
         return;
 
     // re-calculate curve if dimensions changed
-    if (prevGraphW != graphW || prevGraphH != graphH || (int)point.size() != graphW/4)
+    if (prevGraphW != graphW || prevGraphH != graphH || (int)point.size() != graphW-2)
         interpolate ();
 
     double innerW = double(graphW-2);