forcing gnuplot terminal = png to avoid requirement of ghostscript

Chart::Gnuplot uses postscript as default terminal, it implies that
ImageMagick will be used in conjunction to Ghostscript to convert the
output file to the PNG format.

see:
http://metacpan.org/module/Chart::Gnuplot#MECHANISM-OF-THIS-MODULE
we dont need to escape any character when using "png" gnuplot terminal
Releasing version 0.004