set macro
Ang = 'atan((f(($1)+0.001)-f(($1)-0.001))/0.002)'

f(x) = 5*besj1(x)*exp(-x/5)
set samples 21
set xrange [0:10]
set yrange [-1:2.5]
unset key
set term pngcairo enh font "Times, 10" size 480,360
set out "plot_vectors.png"
set label "plot with vectors" at sc 0.4, 0.85 font "Helvetica,18"
set xlabel "Ｘ軸 IPAゴシック" font "IPAGothic,12"
set ylabel "Ｙ軸 IPA明朝" font "IPAMincho,12"
plot '+' using 1:(f($1)):(0.4*cos(@Ang)):(0.4*sin(@Ang)) w vectors lw 2