How can I animate a datafile in gnuplot? - animation

Question

I have a file with the coordinates (x1, y1), (x2, y2) of a double pendulum at an instant of time t. I would like to know how to animate this file.
n t x y
1. 0. 0.3435957874318018 -0.36323812418181006
2. 0. 0.3490554835456995 -0.8632083150115673
1. 0.03 0.3299928793095484 -0.3756390549516834
2. 0.03 0.34657472136383705 -0.8753640218225479
1. 0.06 0.3099940098067639 -0.3923056383534705
2. 0.06 0.3437105896402516 -0.891167535319286
This are the first three positions of the double pendulum, 'n' is the particle number

Answer 1

The following might be a starting point.
Since I don't have enough data for illustration, I generated some data (note: it's not "real" double pendulum data). The assumption is that the anchor point is a at 0,0. Furthermore, it might be desired to not just draw the points with points but also the bars of the pendulum with vectors. It's a bit tricky and not obvious how to draw the second bar since you have to remember the previous position of mass1. I hope you can figure out how to code works.
Code:
### animate double pendulum
reset session
# create some test data (no "real" double pendulum data, just for illustration)
set print $Data
do for [t=0:15] {
print sprintf("%d. %g %g %g", 1, t, x1=cos(t*pi/20+pi/8), y1=-sin(t*pi/20+pi/8))
print sprintf("%d. %g %g %g", 2, t, x2=x1+cos(t/10.), y2=y1-sin(t/10.))
}
set print
set size ratio -1
set xrange[-1.5:2.0]
set yrange[-2:0.1]
set term gif size 400,300 animate delay 20
set output "DoublePendulum.gif"
do for [i=0:15] {
plot $Data u (0):(0):3:4 every ::2*i::2*i w vectors lc "grey" nohead notitle, \
x2=y2=NaN '' u (x1=x2,x2=$3):(y1=y2,y2=$4):(x1-x2):(int($0)%2==0 ? NaN: y1-y2) every ::2*i::2*i+1 w vectors lc "grey" nohead notitle, \
'' u 3:4 every ::2*i::2*i w p pt 7 ps 3 lc "red" title "1", \
'' u 3:4 every ::2*i+1::2*i+1 w p pt 7 ps 2 lc "blue" title "2", \
}
set output
# just for illustration: plot all positions at the same time
set term wxt # or any other terminal
plot $Data u (0):(0):3:4 every 2 w vectors lc "grey" nohead notitle, \
x2=y2=NaN '' u (x1=x2,x2=$3):(y1=y2,y2=$4):(x1-x2):(int($0)%2==0 ? NaN: y1-y2) w vectors lc "grey" nohead notitle, \
'' u 3:4 every 2 w p pt 7 ps 3 lc "red" title "1", \
'' u 3:4 every 2::1 w p pt 7 ps 2 lc "blue" title "2", \
### end of code
Result: (no "real" double pendulum data, just for illustration)
All positions (in wxt terminal):