Control Crosshair position
lazarus / 55-TAChart

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marcou · 2016-06-07 17:04

Hi,

I'd like to set the position of a TDataPointCrosshairTool. I would
like to change its value and then the crosshair to be updated. I could
not figure this out unless editing TAChart to add a write to the
Position property. But then the crosshair does not update. Is there a
way to do it in clean way?

thanks,
Gilles Marcou

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wp · 2016-06-07 17:31

Property TDatapointCrosshairTool.Position?

But it think it will not work to move the Position by code since the
CrosshairCursor must be captured by a data point and the position will
change on the next mouse event.

I don't know what you want to achieve. Maybe it would be easier to use
a TConstantLine series for your purpose.

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marcou · 2016-06-08 06:54

Thanks for your reply. Indeed I was not enough precise.

I'd like to have to charts, each with one serie, but the points of
both chart are refering to the same conceptual objects. So, if a user
click on one chart, I'd like to show him the corresponding point on
the other chart. I used to do it with the deprecated reticule, and it
was very friendly.

You're right that the position of the reticule is lost as soon as a
mouse event is executed on a Chart. However, this is not a real
problem for my application because the user is concerned by the
position of the reticule only when clicking on a Chart. So on mouse
click event it is possible to show or hide the relevant crosshair.

Best regards,
Gilles Marcou

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wp · 2016-06-08 09:17

You mean something like in the attachment?

Here every chart has its own toolset consisting of a datapoint click
tool (for clicking on a data point) and a crosshair tool for visual
feedback of where the click occurs (could be omitted), plus a constant
line series (initially hidden). Both charts share the same x values.
When you click on a data point in one chart the constant line series
is moved to the datapoint with the same x value in the other chart and
shown. When you move the mouse into the other chart the constant line
in this chart disappears to avoid confusion.

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marcou · 2016-06-08 16:42

Hi,

I cannot execute your attachement... There is an unknwon
MouseInsideOnly property of the TDataPointCrossHairTool. I use Lazarus
1.4.4 on mac, but maybe you use a more recent one...

However,your description is globally correct. In my case, x is an
object with properties p1, p2, p3 and p4 and I plot (p1,p2) in one
chart and (p3,p4) in the second one. Beside, my user needs to be able
to select one of the objets into a list, and see where it is located
on both plots.

Indeed, this would be a good workaround. I'll surely try to use your
solution. Beside, the use of CrossHairTool to activate or inactivate
the TConstantLine seems elegant.

In fact, I tried to change the point colors of the serie, but the big
advantage of crosshair, is that it is not affected by the overlap of
data points (as adding an additional serie). And, I like the rendering
too.

Thanks for your help,
Gilles Marcou

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wp · 2016-06-08 16:59

I am using trunk, but I think not much has changed since 1.4.4. Open
the lfm file in a text editor (not the Lazarus IDE) and delete the
offending line with MouseInsideOnly.