TAChart Logarithmic and Linear scale on one Chart
lazarus / 55-TAChart

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markostarko · 2012-07-27 08:25

Hello!

I'am working on project for process control, and have been trying to
plot logarithmic scale and linear scale to one chart. However i made
Chart whit two axis and assign logarithmical transformation to one of
them, but how to scale it propperly. What is AxisindexY property and
how does it work.

Also is it possible to draw vertical line on chart and get Y axis vale
from line position?
Any advice is much appreciated

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wp · 2012-07-27 11:06

I am not quite sure what you mean: Do you, for example, want to have
several curves in the chart, some drawn with a linear scale in the
left axis, and the other ones with a log scale on the right axis?

Have a look at the example in demos\axistransf. Moreover, there are
several postings in this forum (board "Graphics - TAChart") which
refer to logarithmic axis transforms.

You have to assign the AxisIndexY of the log series to the index of
the logarithmic y axis (you see that in the object inspector tree).
And, of course, you need a LogarithmTransform that is assigned to this
axis.

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markostarko · 2012-07-27 13:04

I done that now   but still right axis title is not shown, what about
vertical line that would be cursor for graph and also show current
values. How can i get series value at cursor position from x,y on
mouse move. Would situation change whit logarithmic transform.

If anybody have good example for this situation getting y value from
logarithmic chart on mouse move pleas explain how it's done.

I read everything available on forum, but it's not all clear to me.
It's my first project whit lazarus and at beginning it went so well,
but now whit plotting part i have some difficulties.

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Ask · 2012-07-27 19:27

Quoteright axis title is not shownDid you set Title.Visible = true?

Quote vertical line that would be cursor for graph and also show
current valuesUse TDataPointCrosshairTool. See "tools" demo for an
example.
Alternatively, you can use TDataPointDragTool combined with
TConstantLine --
see "line" demo for an example.

QuoteHow can i get series value at cursor position from x,y on mouse
moveUse ASender.Position property from within the
TDataPointCrosshairTool. OnDraw
event handler to get a graph coordinates of the cursor.
Use Series and PointIndex properties to get the series point nearest
to the cursor position.

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wp · 2012-07-27 21:22

Quoteto get a graph coordinates of the cursor.

Since you are new to TAChart and want to use logarithmic axes you
maybe are not aware that the "graph coordinates" are numbers after
calculating the logarithms. This mean you have to back-transform the
position in order to get the value as displayed on the axis. From the
OnDraw event handler of TDatapointCrosshairTool you can do this, for
example, in the following way -- the result will be transferred to a
label caption:

Code: [Select]procedure TForm1.ChartToolset1DatapointCrosshairTool1Dra
w(ASender:TDatapointCrosshairtool);
var
  series: TCustomChartSeries;
  y: Double;
begin
  if not (ASender.Series) is TCustomChartSeries then
    exit;
  series := TCustomChartSeries(ASender.Series);
  y := series.ParentChart.AxisList[series.AxisIndexY].GraphToAxisY(ASe
nder.Position.Y);
  Label1.Caption := Format('y = %g', [y]);
end;

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markostarko · 2012-07-28 13:23

Thanx for all support, project is almost finished. Lazarus and i are
definetly frends from now on.

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markostarko · 2012-07-28 18:23

Just one more think, do i need new version of Lazarus get
DatapointCrosshairTool,
as i saw from forum it's your contribution.
My question in short where to get the component.

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wp · 2012-07-28 22:17

I would not recommend to merge this component into an existing
installation. Get yourself a current Lazarus snapshot from
http://freepascal.dfmk.hu/test/lazarus/ (this link is at the left-hand
side of this forum). If you are on Windows, these versions are ready
to install. And don't be scared by the message that "you are using
completely untested software" - the snapshots are very stable. Delete
(or rename) the old lazarus folder before installing the snapshot.