p.murrell at auckland.ac.nz
Sun Feb 23 21:00:03 CET 2003
> Why this command does not return grob? What is special about grill and
> what makes it different from axis, lines, rectangles...
Most of the predefined grid drawing functions do two things: draw
something and create and (invisibly) return a grob (graphical object).
This means that they can be used in several different ways:
"procedurally" just for their side effect (drawing something); or
"functionally" for the object they produce (e.g., to use in the
placement or sizing of another object); or both (e.g., to have a record
of what has been drawn that can be uniquely identified and edited). The
default behaviour is "both" so that grid functions appear to work like
their standard graphics counterparts (i.e., they just draw something).
Functions that call the grid drawing functions can choose to just draw
something, or they can choose to use the graphical objects from the grid
functions to produce a graphical object of their own. Ideally, a new
function will do the latter so that functions that want to call it also
have a choice. But it is easier to write a function that just does drawing.
grid.grill is an example of a function that just does drawing. It's a
convenience function that was easy to write. It should be rewritten to
produce a graphical object at some stage -- perhaps a good exercise for
someone learning grid? :)
Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
64 9 3737599 x85392
paul at stat.auckland.ac.nz
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