How to add time dependent labels in Gnuplot - label

Question

A datafile contains three columns with the first and second representing the x & y position of a circle at a time (in seconds) given by the third column. For example, the first two lines in "data.txt" give the position of two circles at time=0, followed by two blank lines then the position of the two circles at time=0.1 sec and so forth. The first few lines of data.txt are:
0 0 0
-1 -1 0
1 1.0 0.1
-1 -0.5 0.1
1.2 1.25 0.2
-0.5 -0.25 0.2
...
The Gnuplot code producing a series of frames (a movie) with the position of the two circles in time is:
set terminal gif size 1200,1200 animate delay 500
set output "movie.gif"
stats "data.txt" u 1:2 name "A"
set style circle radius graph 0.025; set style fill solid
set xrange [A_min_x*1.1:A_max_x*1.1]
set yrange [A_min_y*1.1:A_max_y*1.1]
do for [i=0:A_blocks-2] {
plot "data.txt" index i u 1:2 w circle
}
I'm trying to add a label or textbox of the form "Time=?" to each frame where the question mark is replaced by the number from the third column. Any suggestions on how to do it?

Answer 1

This answer requires the current version of gnuplot (5.4)
Assuming that all the column 3 entries in a given data block are the same, it should be sufficient to say
plot "data.txt" index i u 1:(FOO=strcol(3),column(2)) w circle title sprintf("Time = %s",FOO)
This updates the value of FOO for each line used. The title uses whatever the final update was for that plot. If you must specifically choose the column 3 value from a particular line within the block, or calculate something like an average, that's a harder problem. In that case please clarify.
Edit:
The reason this requires 5.4 is that earlier versions evaluated the title before reading the data for that plot. Version 5.4 evaluates the title after reading the data.

Answer 2

The first (not too obvious) solution which comes to my mind: plot your data in a loop and assign the value of the 3rd column to a variable, e.g. t. Use keyenty to print the legend with your time re-using the variable t. In order to avoid a symbol in the legend use plotting style with points and a point with pointsize 0.
Code:
### animation with time label
reset session
$Data <<EOD
0 0 0
-1 -1 0
1 1.0 0.1
-1 -0.5 0.1
1.2 1.25 0.2
-0.5 -0.25 0.2
EOD
set terminal gif size 400,400 animate delay 100
set output "SO70474478.gif"
stats $Data u 1:2 name "A" nooutput
set style circle radius graph 0.025; set style fill solid
set xrange [A_min_x*1.1:A_max_x*1.1]
set yrange [A_min_y*1.1:A_max_y*1.1]
set key top left
do for [i=0:A_blocks-1] {
plot $Data index i u 1:(t=$3,$2) w circle notitle, \
keyentry w points ps 0 ti sprintf("Time: %.1f",t)
}
set output
### end of code
Result: