diff --git a/components/tachart/tacustomsource.pas b/components/tachart/tacustomsource.pas
index f8e13e5bcd..f37f762c25 100644
--- a/components/tachart/tacustomsource.pas
+++ b/components/tachart/tacustomsource.pas
@@ -828,8 +828,8 @@ begin
         for i:=0 to Count - 1 do
           UpdateMinMax(Item[i]^.X, FBasicExtent.a.X, FBasicExtent.b.X);
     end else begin
-      UpdateMinMax(0, FBasicExtent.a.X, FBasicExtent.b.X);
-      UpdateMinMax(Count-1, FBasicExtent.a.X, FBasicExtent.b.X);
+      FBasicExtent.a.X := 0;
+      FBasicExtent.b.X := Count - 1;
     end;
 
     if YCount > 0 then begin