Scale: Missing Min and Max indication when Chart.Margins.Left/Right=0
lazarus / 55-TAChart

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CM630 · 2021-01-12 16:53

When I place a TAchart on a form and execute the code below the
minimum and maximum values of the X scale (0 and 0,4) are not
displayed.
It occurs that this is caused by Chart.Margins.Left:=0; and
Chart.Margins.Right:=0;. If I set Chart.Margins.Left:=1; and
Chart.Margins.Right:=1; the problem is gone.
Is there a way to have a normal scale with Chart.Margins.Left/Right=0;
?

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.FormCreate(Sender: TObject);begin  Chart.Extent.UseXMax:=True; 
Chart.Extent.UseXMin:=True;  Chart.Extent.UseYMax:=True; 
Chart.Extent.UseYMin:=True;  Chart.Extent.XMin:=0; 
Chart.Extent.XMax:=0.4;  Chart.Extent.YMin:=0; 
Chart.Extent.YMax:=0.4;  Chart.Margins.Top:=0; 
Chart.Margins.Bottom:=0;  Chart.MarginsExternal.Left:=20; 
chart.LeftAxis.Margin:=0;  Chart.LeftAxis.Title.Margins.Left:=0; 
Chart.LeftAxis.Title.Distance:=0;    //Next two problem remove the Mn
and Max values of the X scale.  Chart.Margins.Left:=0; 
Chart.Margins.Right:=0;end;

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wp · 2021-01-12 17:36

Maybe there is some round-off error when the label positions are
calculated. You know: in floating point calculations 0 is not
necessarily equal to 0. It works for me when I set Chart.Extent.XMin =
-1E-12. I cannot comment on Xmax which is working for me out of the
box, but maybe you must apply a tiny offset there too (add it in this
case)

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CM630 · 2021-01-12 20:44

I am aware that float integers are never integers, though I am not
sure if 0 and 2 are the only exceptions.
But for some reason when Chart.Margins.Right/Left are changed zero<->
nonzero the scale is recalculated, and when Chart.Margins.Top/Bottom
are changed the scale is not recalculated, as it can be seen from the
attached snippet.

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wp · 2021-01-12 22:20

Calculation of the scaling factors is a quite complex iterative
process, because when the available chart size changes (for example by
changing Margins), the labels may change and thus change the available
chart size in turn and so on.

Trying to fix this labeling issue is an operation in the heart of
TAChart, and the chance to break something is very high. The last time
I worked in this context I was busy for weeks. So please, either
accept the situation or work around it by expanding the labeled axis
range by a tiny epsilon as explained above.

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CM630 · 2021-01-13 07:55

Thank you for the assistance.Quote from: wp on January 12, 2021,
11:20:28 pm...work around it by expanding the labeled axis range by a
tiny epsilon as explained above...
If I get you right, the tiny epsilon has to be applied for each value
manually, until a result is achieved, so it is not applicable for
dynamically changing scales. So I will stick to
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
Chart.Margins.Left:=1;  Chart.Margins.Right:=1;

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wp · 2021-01-13 08:47

Sorry, I don't understand what you want to achieve. Force the axis to
begin and end at a given value and to place the first/last labels
immediately at the begin/end of the axis? In this case the axis limits
must be input by the user, and I don't see a problem to subtract/add
an epsilon. Here is an example in which the user can enter the axis
limit in edit controls:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i uses 
TAChartUtils; procedure TForm1.Button1Click(Sender: TObject);const 
EPS = 1E-9;var  ext: TDoubleRect;begin  ext.a.x :=
StrToFloat(EditXMin.Text) - EPS;  ext.b.x := StrToFloat(EditXMax.Text)
+ EPS;  ext.a.y := StrToFloat(EditYMin.Text) - EPS;  ext.b.y :=
StrToFloat(EditYMax.Text) + EPS;  Chart1.Extent.FixTo(ext);end;But
this works only when the first and/or last labels are integer
multiples of the interval length that the chart determines. Suppose
the user wants the axis to begin at 0 and to end at 0.39. Then the
final label will not be drawn when the interval length is determined
to be 0.02, 0.05, 0.1, for example.

Automatic labeling is very complex, and will probably fail if you have
certain restrictions in mind. In this case, calculate the labels
yourself and store them in a ChartListSource which you attach to the
Marks.Source of the axis.

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CM630 · 2021-01-14 07:05

I think I've got it now: When Chart.Margins.Left:=1; TaChart considers
Chart.Extent.XMin:=0 for 0 - ε, and when Chart.Margins.Left:=0; it
considers it for 0 + ε. And because Chart.Extent.XMin:=0+ε > 0, the 0
is not shown on the scale, because it is out of it.


I have tried to apply
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   ext.a.x
:= StrToFloat(EditXMin.Text) - EPS;  ext.b.x :=
StrToFloat(EditXMax.Text) + EPS;  ext.a.y := StrToFloat(EditYMin.Text)
- EPS;  ext.b.y := StrToFloat(EditYMax.Text) + EPS; It works for the 0
but not for the 0,4. I suppose that it can be handled with
Chart.BottomAxis.Marks.Source := TListChartSource; but there is
another complication- I have to buffer the desired values of the
extents, because I cannot read them back from the chart.
So,it seems there is a solution, but for now, I will stick to
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
Chart.Margins.Left:=0;  Chart.Margins.Right:=0; Thanks once again!