diff --git a/components/tachart/tacustomsource.pas b/components/tachart/tacustomsource.pas
index a9bb612a52..7f6942d097 100644
--- a/components/tachart/tacustomsource.pas
+++ b/components/tachart/tacustomsource.pas
@@ -655,6 +655,9 @@ begin
         UpdateMinMax(YList[j], Result.a.Y, Result.b.Y);
 end;
 
+// ALB -> leftmost item where X >= AXMin, or Count if no such item
+// ALB -> rightmost item where X <= AXMax, or -1 if no such item
+// If the source is sorted, performs binary search. Otherwise, skips NaNs.
 procedure TCustomChartSource.FindBounds(
   AXMin, AXMax: Double; out ALB, AUB: Integer);
 
@@ -690,10 +693,10 @@ begin
   end
   else begin
     ALB := 0;
-    while (ALB < Count) and (IsNan(Item[ALB]^.X) or (Item[ALB]^.X < AXMin)) do
+    while (ALB < Count) and (NumberOr(Item[ALB]^.X, AXMin - 1) < AXMin) do
       Inc(ALB);
     AUB := Count - 1;
-    while (AUB >= 0) and (IsNan(Item[AUB]^.X) or (Item[AUB]^.X > AXMax)) do
+    while (AUB >= 0) and (NumberOr(Item[AUB]^.X, AXMax + 1) > AXMax) do
       Dec(AUB);
   end;
 end;
@@ -904,8 +907,7 @@ begin
   FValuesTotal := 0;
   for i := 0 to Count - 1 do
     with Item[i]^ do
-      if not IsNan(Y) then
-        FValuesTotal += Y;
+      FValuesTotal += NumberOr(Y);
   FValuesTotalIsValid := true;
   Result := FValuesTotal;
 end;