Many thanks for pngcairo!
Unfortunately it suffers from the problem observed
for eps files too: The rapidly changing lines overshoot
the region they should fill much over their line-width.
See the attached illustration.
The behavior is illustrated by the following script:
set term pngcairo size 1200,800
set output 'xsin1x.png'
set samples 100000
plot [0:0.1][-0.1:0.1] x*sin(1/x) lw 2, x, -x
gnuplot version: 4.4.0, 4.5.0
OS: fedora 12