locator(n=0)

There is nothing in the design of R base graphics to report the current 
position of the mouse without clicking.  Indeed, the graphics model does 
not presume a mouse and pointer (and probably dates from the days of 
cross-hairs manipulated with thumb wheels), and could conceivably use a 
touchscreen, for example.

The current graphics driver model is not really extensible either,
although Paul Murrell is working on improving this. We can't add features
to one or two graphical devices without changing the API and all the
devices (not all of which are in the R sources).

locator(n=1) returns the coordinates of the position of the mouse.
But you have to click the left button of the mouse.
How can I determine the mouse position without any click?
Is it possible to extend locator in a way that  
locator(n=0) outputs the coordinates at once, without any click event?

TclTk allows us to define very nice animations and demonstrations,
see for example:  demo(tkdensity). But sometimes it is much better
to have output and control in one window and to be able to respond
to changes of the cursor position (e.g. to that a bandwidth).

But Tcl/Tk is a different sort of system, designed for interactive windows 
not plotting graphs.  You could do all this in Tcl/Tk (and there is no 
reason not to have an R graphics device in a Tcl/Tk canvas), or in a 
Java-based graphics device.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595