How can I animate a line between two points in GNUplot? - gnuplot

Question

I'm solving the pendulum's equation of motion, and I need to create an animation in GNUplot that shows the evolution of the system.
R4K provides me with the solution point by point, and this solution is stored in a .dat file named 'd' in columns 2 and 3. These are the orders given to GNUplot:
set xrange [-2:2]
set yrange [-2:2]
set pointsize 2
set style line 2 lc rgb '#0060ad' pt 7
set object circle at first 0,0 size scr 0.01 \
fillcolor rgb 'black’ fillstyle solid
do for [ii=1:3762] {
plot 'd.dat' using 2:3 every ::ii::ii linestyle 2
pause 0.002
}
I now want to connect the circle in [0,0] to the moving point plotted from d.dat with a moving line; how can I do it? I found a solution (kind of) here, but still I don't know how to tell GNUplot to search for a certain point in 'd.dat', different every time, and to draw the line between it and the centre.

Answer 1

The solution of user8153 is so far the best. Let me give a trial. If I were you I would have use arrows that update their positions with the moving end dynamically, since the every command loads one point at time. GNUPLOT provides environment variables such as GPVAL_X_MIN, GPVAL_X_MAX, GPVAL_Y_MIN,... that can be useful:
set xrange [-2:2]
set yrange [-2:2]
set pointsize 2
set style line 2 lc rgb '#0060ad' pt 7
set object circle at first 0,0 size scr 0.01 \
fillcolor rgb 'black’ fillstyle solid
do for [ii=1:3762] {
plot 'd.dat' using 2:3 every ::ii::ii linestyle 2
# makes GPVAL_ variables available
xpos = GPVAL_X_MIN; ypos = GPVAL_Y_MIN # or MAX does not matter since you are
# loading one point at time.
unset arrow # Remove the one of the previous graph
set arrow from 0, 0 to xpos, ypos nohead lc rgb 'black'
replot
pause 0.002
}
I have no data to debug this code snippet, but I would have done something along this line. Later versions of GNUPLOT allow to use stats command that avoids replot (replot could have an effect here on the persistence of images of the film but I am not sure). You can also try something along the line
set xrange [-2:2]
set yrange [-2:2]
set pointsize 2
set style line 2 lc rgb '#0060ad' pt 7
set object circle at first 0,0 size scr 0.01 \
fillcolor rgb 'black’ fillstyle solid
do for [ii=1:3762] {
stats 'd.dat' using 2:3 every ::ii::ii # applied the statistical summary of
# the data file
xp = STATS_min_x; yp = STATS_min_y
unset arrow
set arrow from 0, 0 to xp, yp nohead lc rgb 'black'
plot 'd.dat' using 2:3 every ::ii::ii linestyle 2
pause 0.002
}
I have never used STATS with the every command (let me know the output just for fun).
Hope that helps