add ability to restrict yrange in gnuplot example

This can be necessary when the maximum measured latency (for example) is
rather uninteresting and SLAs cannot be visualized because the maximum
sets the scale such that all interesting points lie directly on the
x-axis.

Also enlarge the PNG size to get more information onto the screen.
Merge pull request #49 from rkuhn/wip-gnuplot-maxvalue

add ability to restrict yrange in gnuplot example