diff --git a/components/tachart/demo/fit/Main.lfm b/components/tachart/demo/fit/Main.lfm
index 918c8a1c73..883733025c 100644
--- a/components/tachart/demo/fit/Main.lfm
+++ b/components/tachart/demo/fit/Main.lfm
@@ -13,18 +13,18 @@ object frmMain: TfrmMain
     Left = 8
     Height = 527
     Top = 8
-    Width = 400
+    Width = 440
     Align = alLeft
     BorderSpacing.Around = 8
     BevelOuter = bvNone
     ClientHeight = 527
-    ClientWidth = 400
+    ClientWidth = 440
     TabOrder = 0
     object PageControl1: TPageControl
       Left = 0
       Height = 527
       Top = 0
-      Width = 400
+      Width = 440
       ActivePage = TabSheet1
       Align = alClient
       TabIndex = 0
@@ -32,18 +32,18 @@ object frmMain: TfrmMain
       object TabSheet1: TTabSheet
         Caption = 'Preparation'
         ClientHeight = 499
-        ClientWidth = 392
+        ClientWidth = 432
         object gbDataGeneration: TGroupBox
           Left = 4
           Height = 157
           Top = 4
-          Width = 384
+          Width = 424
           Align = alTop
           AutoSize = True
           BorderSpacing.Around = 4
           Caption = ' Data generation '
           ClientHeight = 137
-          ClientWidth = 380
+          ClientWidth = 420
           Font.Style = [fsBold]
           ParentFont = False
           TabOrder = 0
@@ -51,7 +51,7 @@ object frmMain: TfrmMain
             AnchorSideTop.Control = edNoiseY
             AnchorSideRight.Control = cbTestFunction
             AnchorSideRight.Side = asrBottom
-            Left = 349
+            Left = 389
             Height = 22
             Hint = 'Export data to verify fit with other software'
             Top = 31
@@ -105,7 +105,7 @@ object frmMain: TfrmMain
             Height = 23
             Hint = 'Select a dataset for fitting'
             Top = 4
-            Width = 272
+            Width = 312
             Anchors = [akTop, akLeft, akRight]
             BorderSpacing.Left = 16
             BorderSpacing.Top = 4
@@ -208,7 +208,7 @@ object frmMain: TfrmMain
             Left = 8
             Height = 8
             Top = 89
-            Width = 364
+            Width = 404
             Anchors = [akTop, akLeft, akRight]
             BorderSpacing.Left = 8
             BorderSpacing.Right = 8
@@ -240,11 +240,11 @@ object frmMain: TfrmMain
             Left = 93
             Height = 30
             Top = 97
-            Width = 287
+            Width = 327
             Anchors = [akTop, akLeft, akRight]
             BorderSpacing.Left = 16
             BorderSpacing.Bottom = 4
-            Caption = 'x, y : two columns, separator semicolon or tab'#13#10'dy: in optional third column'
+            Caption = 'x, y : two columns, separated by comma, semicolon or tab'#13#10'dy: in optional third column'
             ParentColor = False
             ParentFont = False
           end
@@ -253,36 +253,36 @@ object frmMain: TfrmMain
           Left = 4
           Height = 330
           Top = 169
-          Width = 384
+          Width = 424
           Align = alClient
           BorderSpacing.Left = 4
           BorderSpacing.Top = 8
           BorderSpacing.Right = 4
           Caption = ' Fitting '
           ClientHeight = 310
-          ClientWidth = 380
+          ClientWidth = 420
           Font.Style = [fsBold]
           ParentFont = False
           TabOrder = 1
           object gbFitRange: TGroupBox
-            AnchorSideLeft.Control = GroupBox1
-            AnchorSideTop.Control = GroupBox1
+            AnchorSideLeft.Control = gbFixedParams
+            AnchorSideTop.Control = gbFixedParams
             AnchorSideTop.Side = asrBottom
-            AnchorSideRight.Control = GroupBox1
+            AnchorSideRight.Control = gbFixedParams
             AnchorSideRight.Side = asrBottom
             Left = 14
             Height = 109
             Top = 151
-            Width = 352
+            Width = 392
             Anchors = [akTop, akLeft, akRight]
             AutoSize = True
             BorderSpacing.Top = 16
             Caption = 'Fit range'
             ClientHeight = 89
-            ClientWidth = 348
+            ClientWidth = 388
             Font.Style = [fsBold]
             ParentFont = False
-            TabOrder = 0
+            TabOrder = 3
             object cbFitRangeUseMin: TCheckBox
               AnchorSideLeft.Control = gbFitRange
               AnchorSideTop.Control = gbFitRange
@@ -309,7 +309,7 @@ object frmMain: TfrmMain
               Caption = 'Use maximum'
               OnClick = cbFitRangeUseMaxClick
               ParentFont = False
-              TabOrder = 1
+              TabOrder = 2
             end
             object edFitRangeMin: TFloatSpinEdit
               AnchorSideLeft.Control = cbFitRangeUseMin
@@ -326,7 +326,7 @@ object frmMain: TfrmMain
               MinValue = -1E308
               OnChange = edFitRangeMinChange
               ParentFont = False
-              TabOrder = 2
+              TabOrder = 1
               Value = 5
               Visible = False
             end
@@ -377,7 +377,6 @@ object frmMain: TfrmMain
             Width = 53
             BorderSpacing.Left = 16
             BorderSpacing.Top = 8
-            Enabled = False
             OnChange = edFitOrderChange
             ParentFont = False
             TabOrder = 1
@@ -392,7 +391,6 @@ object frmMain: TfrmMain
             Top = 39
             Width = 114
             Caption = 'Degree of polynomial'
-            Enabled = False
             ParentColor = False
             ParentFont = False
           end
@@ -406,7 +404,7 @@ object frmMain: TfrmMain
             Height = 23
             Hint = 'Select a function type to be used for fitting to the generated data'
             Top = 4
-            Width = 277
+            Width = 317
             Anchors = [akTop, akLeft, akRight]
             BorderSpacing.Left = 12
             BorderSpacing.Top = 4
@@ -422,7 +420,7 @@ object frmMain: TfrmMain
             OnSelect = cbFitEquationSelect
             ParentFont = False
             Style = csDropDownList
-            TabOrder = 2
+            TabOrder = 0
           end
           object lblFitEquation: TLabel
             AnchorSideLeft.Control = gbFitting
@@ -437,7 +435,7 @@ object frmMain: TfrmMain
             ParentColor = False
             ParentFont = False
           end
-          object GroupBox1: TGroupBox
+          object gbFixedParams: TGroupBox
             AnchorSideLeft.Control = lblFitEquation
             AnchorSideTop.Control = edFitOrder
             AnchorSideTop.Side = asrBottom
@@ -446,16 +444,16 @@ object frmMain: TfrmMain
             Left = 14
             Height = 61
             Top = 74
-            Width = 352
+            Width = 392
             Anchors = [akTop, akLeft, akRight]
             AutoSize = True
             BorderSpacing.Top = 16
             Caption = 'Fixed parameters (check to keep constant)'
             ClientHeight = 41
-            ClientWidth = 348
-            TabOrder = 3
+            ClientWidth = 388
+            TabOrder = 2
             object cbFitParam0Fixed: TCheckBox
-              AnchorSideLeft.Control = GroupBox1
+              AnchorSideLeft.Control = gbFixedParams
               AnchorSideTop.Control = edFitParam0
               AnchorSideTop.Side = asrCenter
               Left = 16
@@ -464,14 +462,14 @@ object frmMain: TfrmMain
               Width = 44
               BorderSpacing.Left = 16
               Caption = 'b0 ='
-              OnChange = FixedParamsChanged
+              OnChange = cbFitParam0FixedChange
               ParentFont = False
               TabOrder = 0
             end
             object edFitParam0: TFloatSpinEdit
               AnchorSideLeft.Control = cbFitParam0Fixed
               AnchorSideLeft.Side = asrBottom
-              AnchorSideTop.Control = GroupBox1
+              AnchorSideTop.Control = gbFixedParams
               Left = 64
               Height = 23
               Top = 4
@@ -479,6 +477,7 @@ object frmMain: TfrmMain
               BorderSpacing.Left = 4
               BorderSpacing.Top = 4
               BorderSpacing.Bottom = 14
+              Enabled = False
               MaxValue = 1E308
               MinValue = -1E308
               OnChange = FixedParamsChanged
@@ -488,26 +487,27 @@ object frmMain: TfrmMain
             object cbFitParam1Fixed: TCheckBox
               AnchorSideTop.Control = cbFitParam0Fixed
               AnchorSideRight.Control = edFitParam1
-              Left = 212
+              Left = 252
               Height = 19
               Top = 6
               Width = 44
               Anchors = [akTop, akRight]
               Caption = 'b1 ='
-              OnChange = FixedParamsChanged
+              OnChange = cbFitParam1FixedChange
               ParentFont = False
               TabOrder = 2
             end
             object edFitParam1: TFloatSpinEdit
               AnchorSideTop.Control = edFitParam0
-              AnchorSideRight.Control = GroupBox1
+              AnchorSideRight.Control = gbFixedParams
               AnchorSideRight.Side = asrBottom
-              Left = 256
+              Left = 296
               Height = 23
               Top = 4
               Width = 80
               Anchors = [akTop, akRight]
               BorderSpacing.Right = 12
+              Enabled = False
               MaxValue = 1E308
               MinValue = -1E308
               OnChange = FixedParamsChanged
@@ -520,7 +520,7 @@ object frmMain: TfrmMain
             AnchorSideTop.Side = asrBottom
             AnchorSideRight.Control = cbFitEquation
             AnchorSideRight.Side = asrBottom
-            Left = 312
+            Left = 352
             Height = 23
             Top = 272
             Width = 54
@@ -540,7 +540,7 @@ object frmMain: TfrmMain
             AnchorSideTop.Control = edConfLevel
             AnchorSideTop.Side = asrCenter
             AnchorSideRight.Control = edConfLevel
-            Left = 220
+            Left = 260
             Height = 15
             Top = 276
             Width = 88
@@ -576,21 +576,21 @@ object frmMain: TfrmMain
     end
   end
   object pnlChart: TPanel
-    Left = 429
+    Left = 469
     Height = 527
     Top = 8
-    Width = 589
+    Width = 549
     Align = alClient
     BorderSpacing.Around = 8
     BevelOuter = bvNone
     ClientHeight = 527
-    ClientWidth = 589
+    ClientWidth = 549
     TabOrder = 1
     object Chart: TChart
       Left = 0
       Height = 485
       Top = 0
-      Width = 589
+      Width = 549
       AxisList = <      
         item
           Grid.Visible = False
@@ -698,12 +698,12 @@ object frmMain: TfrmMain
       Left = 0
       Height = 42
       Top = 485
-      Width = 589
+      Width = 549
       Align = alBottom
       AutoSize = True
       BevelOuter = bvNone
       ClientHeight = 42
-      ClientWidth = 589
+      ClientWidth = 549
       TabOrder = 1
       object cbLogX: TCheckBox
         AnchorSideLeft.Control = pnlLog
@@ -799,7 +799,7 @@ object frmMain: TfrmMain
     end
   end
   object Splitter1: TSplitter
-    Left = 416
+    Left = 456
     Height = 543
     Top = 0
     Width = 5
diff --git a/components/tachart/demo/fit/Main.pas b/components/tachart/demo/fit/Main.pas
index de5ba2a3dc..1ccd7176fe 100644
--- a/components/tachart/demo/fit/Main.pas
+++ b/components/tachart/demo/fit/Main.pas
@@ -41,7 +41,7 @@ type
     ChartAxisTransformations: TChartAxisTransformations;
     edFitParam0: TFloatSpinEdit;
     edFitParam1: TFloatSpinEdit;
-    GroupBox1: TGroupBox;
+    gbFixedParams: TGroupBox;
     Label1: TLabel;
     lblNoiseY1: TLabel;
     LogarithmAxisTransform: TLogarithmAxisTransform;
@@ -75,6 +75,8 @@ type
     procedure cbCombinedExtentChange(Sender: TObject);
     procedure cbDrawFitRangeOnlyClick(Sender: TObject);
     procedure cbFitEquationSelect(Sender: TObject);
+    procedure cbFitParam0FixedChange(Sender: TObject);
+    procedure cbFitParam1FixedChange(Sender: TObject);
     procedure cbHTMLChange(Sender: TObject);
     procedure cbShowConfidenceIntervalsChange(Sender: TObject);
     procedure cbShowErrorbarsChange(Sender: TObject);
@@ -263,6 +265,18 @@ begin
   end;
 end;
 
+procedure TfrmMain.cbFitParam0FixedChange(Sender: TObject);
+begin
+  edFitParam0.Enabled := cbFitParam0Fixed.Checked;
+  FixedParamsChanged(Sender);
+end;
+
+procedure TfrmMain.cbFitParam1FixedChange(Sender: TObject);
+begin
+  edFitParam1.Enabled := cbFitParam1Fixed.Checked;
+  FixedParamsChanged(Sender);
+end;
+
 procedure TfrmMain.cbHTMLChange(Sender: TObject);
 begin
   if cbHTML.Checked then Chart.Legend.TextFormat := tfHTML else Chart.Legend.TextFormat := tfNormal;
diff --git a/components/tachart/tafuncseries.pas b/components/tachart/tafuncseries.pas
index 09dd5ede88..11270aeccd 100644
--- a/components/tachart/tafuncseries.pas
+++ b/components/tachart/tafuncseries.pas
@@ -2148,7 +2148,7 @@ begin
   Result := NaN;
   if (FState = fpsValid) and Assigned(FFitStatistics) then begin
     sig2 := FFitStatistics.VarCovar[AIndex, AIndex];
-    if not IsNaN(sig2) then
+    if not IsNaN(sig2) and (sig2 >= 0) then
       Result := sqrt(sig2);
   end;
 end;