piuparts-report - Fix breakage by upgrading python-rpy to python-rpy2.

Recent changes in sid for 'R' and python-rpy have broken chart creation.
There are two problems, 1) python-rpy, a legacy package, will not import
after the upgrade, and 2) some of the 'r' commands are expecting to be
interacting with an X frame buffer.

This commit restores charts by 1) porting the chart creation code to
python-rpy2, and 2) removing the offending commands. Charts are being
created now, but since one of the removed commands is palette(), they
are pretty seriously ugly.
piuparts-report - Use xvfb X buffer to get chart 'R' calls working.

Per http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=705847#55 ,
create an X11 virtual framebuffer for 'R' to use as it wishes while
creating the chart.

It appears that piuparts-report has picked up a segfault at the end
of its run, possibly from this change.
piuparts-report - Add the 'R' palette command back in for charts.

With the X11 virtual framebuffer established, this can be safely
added back in.