[Rd] Plot elements echo NULL (PR#7466)

[andrewr at uidaho.edu]

Full_Name: Andrew Robinson
Version: 2.0.1
OS: FreeBSD
Submission from: (NULL) (211.28.168.242)


Certain plot elements echo NULL when they are set.  For example,

> plot(1:10,1:10)
> axis(1)
NULL
> mtext("test")
NULL
> 

In previous versions the functions were silent.