Generating mutilple plots from GNU in c

I've got many files having data to plot in a single gnu window using c. My current code is

gnuplotPipe = popen ("gnuplot -persistent", "w");
//loop starts for each file
system("gnuplot -p -e \"plot 'file_variable'""); // skipping some steps to generate variable file name
//end loop
fclose(gnuplotPipe);

It is generating graphs in individual windows. How to combine all of them in one?

Files format are like

2 0.000003
2 0.000002
2 0.000002

in file_2.txt

3 0.000001
3 0.000000
3 0.000001

in file_3.txt

.
.
.

in file_n.txt n is between 3 and 98

99 0.004800
99 0.004752
99 0.004716

in file_99.txt.

Any help would be appreciated.