[Rd] multiple Labels in .png plots (PR#7733)

[wb at arb-phys.uni-dortmund.de]

In R-2.0.1 on FreeBSD-5.3, I tried to make multiple plots in PNG format,
using the default plot()-function
with different labels on the y axis. Labels on the 2nd and later pages
overwrite the first instead of replacing it. This is probably the
case for other sorts of lables in PNG also. In Postscript, however,
the working is as expected.

W.B.Kloke