How do I grab values from datafile to make legend?

I want to use system call(s) to grab values from first data line in file file and print into legend of plot.

The command returns a syntax error. I admit come confusion about a- the use of system calls and syntax, despite reading few advanced questions here.

This is what I have:

gnuplot> plot for [i=20:30:1] '4He-processed' index i u 8:($22>0.2&&$22<2?$9:1/0):10 w yerr t system("head -2 4He-processed | tail -1 | awk '{printf "%s %8.3f %8.3f %s" , "'", $3, $4,"'"}'")

with the response: ')' expected with the pointer at the "f" in printf.

I want to have the values in $3 and $4 written to the legend.

This alternate command

gnuplot> plot for [i=20:30:1] '4He-processed' index i u 8:($22>0.2&&$22<2?$9:1/0):10 w yerr t system("head -2 4He-processed | tail -1 ")

puts the entire first line, of each index loop, to the legend

It likely has to do with syntax?

I want the values from $3 and $4, not the column headings:

Here is the some lines (but not all the columns) from the file

nz na e0 theta nu xsect ert y fy fye

2    4    0.150   60.000    0.025   0.330E+02   0.752E+00   -0.0459   0.956E+00   0.218E-01 
2    4    0.150   60.000    0.030   0.497E+02   0.784E+00   -0.0001   0.146E+01   0.230E-01 
2    4    0.150   60.000    0.035   0.483E+02   0.766E+00    0.0315   0.144E+01   0.229E-01 
2    4    0.150   60.000    0.040   0.408E+02   0.728E+00    0.0573   0.125E+01   0.224E-01 

This continues for many blocks. Here, if I were to start my loop with the first block, the values (at $3 and $4) would be 0.150 and 60.000 which correspond to the energy and angle of the projectile and they would hopefully appear in the legend. The plotted quantities ($8,$22 and $23) not pasted here (too many columns).