gnuplot: how to plot sum of three data sets - plot

Question

I have a data in file which I would like to plot using gnuplot. In the file, there are 3 data sets separated by two blank lines so that gnuplot can differentiate between the data sets by 'index'. I can plot three data sets separately via 'index' option of 'plot' command.
However, I am not sure how can I plot the data which is sum of 2nd column of all three data sets?
Note: all three data sets have same x data, i.e. 1st column

Answer 1

To do this the simplest thing would be to change your file format. Gnuplot manipulates columns pretty well. Since you are sharing the x data, you can change the file format to have four columns (assuming you are just plotting (x,y) data):
<x data> <y1 data> <y2 data> <y3 data>
and use a command like
plot 'data.dat' using 1:2 title 'data 1', \
'' u 1:3 t 'data 2', \
'' u 1:4 t 'data 3', \
'' u 1:($2+$3+$4) t 'sum of datas'
The dollar signs inside the parens in the using column specification allow you to add/subtract/perform other functions on columnar data.
This way your data file will also be smaller since you won't repeat the x data.

Answer 2

#Youjun Hu, never say that there is "no way" to do something with gnuplot. Most of the cases there is a way with gnuplot only, sometimes maybe not obvious or sometimes a bit cumbersome.
Data: SO16861334.dat
1 11
2 12
3 13
4 14
1 21
2 22
3 23
4 24
1 31
2 32
3 33
4 34
Code 1: (works with gnuplot 4.6.0, needs some adaptions for >=4.6.5)
In gnuplot 4.6.0 (version at the time of OP's question) there were no datablocks and no plot ... with table. The example below only works for 3 subdatasets, but could be adapted for other numbers. However, arbitrary large number of subdatasets will be difficult with this approach.
### calculate sum from 3 different (sub)datasets, gnuplot 4.6.0
reset
FILE = "SO16861334.dat"
stats FILE u 0 nooutput
N = int(STATS_records/STATS_blocks) # get number of lines per subblock
set table FILE."2"
plot FILE u 1:2
set table FILE."3"
x1=x2=y1=y2=NaN
myValueX(col) = (x0=x1,x1=x2,x2=column(col), r=int($0-2)/N, r<1 ? x0 : r<2 ? x1 : x2)
myValueY(col) = (y0=y1,y1=y2,y2=column(col), r<1 ? y0 : r<2 ? y1 : y2)
plot FILE."2" u (myValueX(1)):(myValueY(2))
unset table
set key top left
set offset graph 0.1, graph 0.1, graph 0.2, graph 0.1
plot for [i=0:2] FILE u 1:2 index i w lp pt 7 lc i+1 ti sprintf("index %d",i), \
FILE."3" u 1:2 every ::2 smooth freq w lp pt 7 lc rgb "magenta" ti "sum"
### end of code
Code 2: (works with gnuplot>=5.0.0)
This code works with arbitrary number of subdatasets.
### calculate sum from 3 different (sub)datasets, gnuplot>=5.0.0
reset
FILE = "SO16861334.dat"
set table $Data2
plot FILE u 1:2 w table
unset table
set key top left
set offset graph 0.1, graph 0.1, graph 0.2, graph 0.1
set colorsequence classic
plot for [i=0:2] FILE u 1:2 index i w lp pt 7 lc i+1 ti sprintf("index %d",i), \
$Data2 u 1:2 smooth freq w lp pt 7 lc rgb "magenta" ti "sum"
### end of code
Result: (same result for Code1 with gnuplot 4.6.0 and Code2 for gnuplot 5.0.0)