Level 2.3 : y = -x*cos(x)

# 作図方法
gnuplot<<EOF
clear
plot -x*cos(x)
set grid
set xlabel "x"
set ylabel "y"
set title "y = -x*cos(x)"
set terminal png
set output "func2_3.png"
replot
set terminal pdf
set output "func2_3.pdf"
replot
EOF