Set image size with "set terminal size", not "set size".

Recent versions of gnuplot understand "set size" as the amount of
space to use within the canava size. "set size 1,0.5" was therefore
creating large images with the top half being blank.

set terminal size works as expected at least with gnuplot 4.4 and 4.2.

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>

gitstats

 import time
 import zlib
 
-GNUPLOT_COMMON = 'set terminal png transparent\nset size 1.0,0.5\n'
+GNUPLOT_COMMON = 'set terminal png transparent size 640,240\nset size 1.0,1.0\n'
 ON_LINUX = (platform.system() == 'Linux')
 WEEKDAYS = ('Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun')