diff --git a/components/tachart/tamultiseries.pas b/components/tachart/tamultiseries.pas
index 1399cfe299..3876f1924e 100644
--- a/components/tachart/tamultiseries.pas
+++ b/components/tachart/tamultiseries.pas
@@ -279,7 +279,6 @@ end;
 
 function TBoxAndWhiskerSeries.Extent: TDoubleRect;
 var
-  i, j: Integer;
   x: Double;
 
   function ExtraWidth(AIndex: Integer): Double;
@@ -288,13 +287,8 @@ var
   end;
 
 begin
-  Result := EmptyExtent;
-  if Source.YCount < 5 then exit;
-  Result := inherited Extent;
-  for i := 0 to Count - 1 do
-    with Source[i]^ do
-      for j := 0 to High(YList) do
-        UpdateMinMax(YList[j], Result.a.Y, Result.b.Y);
+  if Source.YCount < 5 then exit(EmptyExtent);
+  Result := Source.ExtentList;
   // Show first and last boxes fully.
   x := GetGraphPointX(0);
   Result.a.X := Min(Result.a.X, x - ExtraWidth(0));
diff --git a/components/tachart/tasources.pas b/components/tachart/tasources.pas
index c2fe817793..3a4736c480 100644
--- a/components/tachart/tasources.pas
+++ b/components/tachart/tasources.pas
@@ -67,7 +67,8 @@ type
     function IsUpdating: Boolean; inline;
   public
     class procedure CheckFormat(const AFormat: String);
-    function Extent: TDoubleRect; virtual;
+    function Extent: TDoubleRect;
+    function ExtentList: TDoubleRect;
     procedure FindBounds(AXMin, AXMax: Double; out ALB, AUB: Integer);
     function FormatItem(const AFormat: String; AIndex: Integer): String;
     function IsSorted: Boolean; virtual;
@@ -359,6 +360,17 @@ begin
   Result := FExtent;
 end;
 
+function TCustomChartSource.ExtentList: TDoubleRect;
+var
+  i, j: Integer;
+begin
+  Result := Extent;
+  for i := 0 to Count - 1 do
+    with Item[i]^ do
+      for j := 0 to High(YList) do
+        UpdateMinMax(YList[j], Result.a.Y, Result.b.Y);
+end;
+
 procedure TCustomChartSource.FindBounds(
   AXMin, AXMax: Double; out ALB, AUB: Integer);