# [R] Why points() is defined specially for a 1 by 2 matrix?

Ben Bolker bolker at ufl.edu
Mon Oct 19 06:15:47 CEST 2009

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Peng Yu wrote:
>
> On Sun, Oct 18, 2009 at 10:26 PM, Richard M. Heiberger <rmh at temple.edu>
> wrote:
>>> points(x[4,],pch=2)# this is plotted as two points
>>
>> drops what it sees as an unnecessary dimension.
>>
>> Use
>>
>>> points(x[4,, drop=FALSE], pch=2)
>>
>> See FAQ 7.5
>>
>> tmp <- matrix(1:2)
>> tmp
>> tmp[,1]
>> tmp[,1,drop=FALSE]
>
> Can I specify 'drop' to FALSE by default so that I don't have to
> specify it explicitly?
>
>

Not that I know of, but things will be easier with the following idiom:

> x = cbind(1:4,3:6)
> plot(x[,1],x[,2],pch=rep(1:2,c(3,1)))

even easier if the plot types correspond to a factor variable in
a data frame:

dat = data.frame(x=1:4,y=3:6,type=factor(c(1,1,1,2)))
with(dat,plot(x,y,pch=as.numeric(type)))

very powerful, but base graphics are (a) easier to get your head around
and (b) easier to adjust if you don't like the defaults.  Changing things
just a little bit in ggplot can be difficult (as an example, the answer to
your other question about getting rid of grid lines has to do with
theme_blank(), something like +options(grid.panel.minor=theme_blank())
[try googling theme_blank for a few examples])

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