デフォルトのパレットが変わったので今すぐにでも gnuplot-5.0 にアップグレードすべき ref: http://qiita.com/xr0038/items/88d55a20efa68604bc2d

.gnuplot

 if ((GPVAL_VERSION < 4.5) \
 ||  (!strstrt(GPVAL_COMPILE_OPTIONS,"+USER_LINETYPES"))) \
     exit
 set linetype 1 lc rgb "dark-violet" lw 2 pt 0
 set linetype 2 lc rgb "sea-green"   lw 2 pt 7
 set linetype 3 lc rgb "cyan"        lw 2 pt 6 pi -1
 set linetype 4 lc rgb "dark-red"    lw 2 pt 5 pi -1
 set linetype 5 lc rgb "blue"        lw 2 pt 8
 set linetype 6 lc rgb "dark-orange" lw 2 pt 3
 set linetype 7 lc rgb "black"       lw 2 pt 11
 set linetype 8 lc rgb "goldenrod"   lw 2
 set linetype cycle 8

file1.txt

cd path/to/hoge/directory
tar zxvf ~/Downloads/gnuplot-5.0.0.tar.gz

file2.txt

sudo apt-get install libreadline6-dev
sudo apt-get install libgd2-xpm-dev libpango1.0-dev
sudo apt-get install libcairo2-dev libwxgtk2.8-dev 

file3.txt

cd gnuplot-5.0.0
./configure --with-readline=gnu
make -j 4
sudo make install

plot.gp

set terminal wxt font ",12"
set xr [0.1:10]; set xtics format "%3.1f"
set yr [0.1:10]; set ytics format "%3.1f"
set log x; set log y
set key bottom right 
set grid x y mx my
plot for [n=1:8] n*x lw 3 t sprintf("%dth-plot",n)