set xrange [0:pi]
set yrange [0:pi]
set iso 20
set samp 20
unset key
a=0.5
set object 1 circle front at graph 0,0 radius 0.1 fc rgb "grey10" lw 1
plot '++' using 1:2:(-a*sin($1)*cos($2)):(a*cos($1)*sin($2))\
          w vec size  .06, 15 filled

pause 5