Fix regexp in test_plot.rb for Ruby1.8

Hi!

The ordering of parameters with Ruby1.8 is not deterministic, because it
comes from a Hash. The previous regexps required two spaces between
'circle' and the other keyword, which does not work if the keyword comes
just after 'circle'.

This problem caused a build failure for the Debian package:
http://bugs.debian.org/713168

Removing one space in these regexp solves the problem.

Cheers,

Cédric