unset border; unset tics; unset key;
set term gif animate delay 20
set output "trace.gif"
set xrange [0:30]
set yrange [0:4]
set object 1 circle front at 4.0,2.0 size 1.0 fillcolor rgb "black" lw 2


do for [ts=0:300] {
    plot "particles_".ts.".dat" with points pointtype 7
}
unset output