and a dialog box is opened over the graph in the center of the screen. In
order to look at the whole graph I am obliged to drag the box in a corner of
the screen with the mouse. It is very tedious and I would like to know if
there is a possibility to act on the coordinates and the dimensions of this
dialog box before it appears on the screen.

Unfortunately, there is no way to position the dialog boxes created by LabTalk.
If you look in the ORIGIN.INI file, you will see a section [WinPos]. This is
where the position of all positionable windows are stored.
I will let the developers know of this suggestion and hopefully it will be
implemented in a future version.