Hello,
The test in check_diagram is

      if ver =~ /dot\s+version(?:\s+gviz)?\s+(\d+)\.(\d+)/
        ok = ($1.to_i > 1) || ($1.to_i == 1 && $2.to_i >= 8)
      end

But the version string that looked like "dot version 2.8 (Sun Aug 13
01:37:46 UTC 2006)" is now "dot - Graphviz version 2.12 (Wed Mar  7
21:41:22 UTC 2007)"

So, the first \s needs to be changed