[Rd] interaction.plot

[Christoph Buser]

Dear R core team

I've a proprosal to improve the function interaction.plot. It
should be allowed to use type = "b". This can be done by
changing the function's header from

function(        , type = c("l", "p"),     )

to

function(        , type = c("l", "p", "b"),     )

Then it works. This type = "b" is useful, if the second level
of the x.factor is missing for some level of the
trace.factor. With type= "l" you loose first level of the
x.factor, too (because you can't draw the line to the missing
second level). With type = "p" so see this first level, but you
have no lines at all (just chaos with points). With type = "b",
you get all existing levels plus the lines between two
contiguous levels (if they both exist).


There is a second point. Using interaction.plot with the
additional argument main creates a warning:

parameter "main" couldn't be set in high-level plot() function 

The reason is that "..." is used twice inside of
interaction.plot, in fact in

matplot( ,...)

and in

axis( ,...)

axis can do nothing with this argument main and creates this
warning. You could replace ,... in the axis function by
inserting all reasonable arguments of axis in the functions
header (interaction.plot) and give over those arguments to
axis. Then you shouldn't get this warning anymore.

Best regards

Christoph Buser

-- 
Christoph Buser <buser at stat.math.ethz.ch>
Seminar fuer Statistik, LEO D6
ETH (Federal Inst. Technology)	8092 Zurich	 SWITZERLAND
phone: x-41-1-632-3505		fax: 632-1228
http://stat.ethz.ch/~buser/