One issue I've noticed is that the colour choices for some of the lines are poor; in particular lindagordinho seems to be rendered white-on-white; some of the others are very close to white. This is automatic within gnuplot and a bit annoying that the png terminal's defaults do this. 
I think there's a work-around, specifying linetype explictly and skipping values in the list which don't render well. It's really the sort of thing that the authors of gnuplot should have sorted out a long time ago, IMO
You'll have to specify the colours and point styles yourself:-
From:
Gnuplot# POINT SIZE AND TYPE
# pointsize is to expand points
set pointsize 2.5
# type 'test' to see the colors and point types available
# lt is for color of the points: -1=black 1=red 2=grn 3=blue 4=purple 5=aqua 6=brn 7=orange 8=light-brn
# pt gives a particular point type: 1=diamond 2=+ 3=square 4=X 5=triangle 6=*
# postscipt: 1=+, 2=X, 3=*, 4=square, 5=filled square, 6=circle,
# 7=filled circle, 8=triangle, 9=filled triangle, etc.
# LINE COLORS, STYLES
# type 'test' to see the colors and point types available.
# Differs from x11 to postscript
# lt chooses a particular line type: -1=black 1=red 2=grn 3=blue 4=purple 5=aqua 6=brn 7=orange 8=light-brn
# lt must be specified before pt for colored points
# for postscipt -1=normal, 1=grey, 2=dashed, 3=hashed, 4=dot, 5=dot-dash
# lw chooses a line width 1=normal, can use 0.8, 0.3, 1.5, 3, etc.
# ls chooses a line style
plot sin(x)k with linespoints lt 2 pt 4
