Multiple data blocks, changing plot title - gnuplot

Question

I'm trying to build a graph with a csv file.
It's supposed to have an undetermined number of data blocks inside.
My CSV looks like this :
year;amount;NAME1
year;amount;NAME1
year;amount;NAME1
year;amount;NAME2
year;amount;NAME2
year;amount;NAME2
So I want my graph to have two curves (or more if there's more blocks), one named NAME1 and the other NAME2.
The only way I've fount to retrieve the name is by using:
title columnhead(3)
But by using this, the first line of my csv is missing, and I can't figure why ...
Here's my script generating the image
gnuplot <<EOF
set terminal png
set title "Stages par professeur par années"
set output "stages_entr_ann.png"
set auto x
set key on outside left bmargin
set datafile separator ";"
set xtics 1
set ytics 1
stats 'fichier3_t.stat'
plot for [IDX=0:STATS_blocks-1] 'fichier3_t.stat' index IDX u 1:2 title columnhead(3) with linespoints ls IDX
EOF
(There's an unknow amount of blocks, so I'm using STATS_blocks)

Answer 1

The point is that columnhead expects something like this:
Year Amount Name
2013 5000 John
2014 8000 Max
2015 12000 Susanne
i.e. the first row of each colum is treated as label, not as data. While gnuplot extracts the name fine, it ignores the rest of the line.
There is no simple and direct solution for this, but you can do a workaround:
plot for [IDX=0:STATS_blocks-1] 'fichier3_t.stat' index IDX u 1:(1/0) title columnhead(3) with linespoints ls IDX, for [IDX=0:STATS_blocks-1] 'fichier3_t.stat' index IDX u 1:(1/0) notitle with linespoints ls IDX
To make it more clear:
plot 'fichier3_t.stat' ... u 1:(1/0) title columnhead(3) with linespoints ..., \
'fichier3_t.stat' ... u 1:2 notitle with linespoints ...
The first command is similar to yours, it just does not plot any data (because 1/0 is always invalid). It's only there to generate the entry in the legend (key).
The second command plots the data, but does not generate any entry in the legend. Just make sure the same line style is assigned to the two plots.
If the table is large and speed is a problem, you may also use every for the title generating command to reduce the number of points it trys to plot.
Finally, you may also get the message "No valid data in xrange" or similar. That's intended here but shouldn't be a problem.