My example source file (two curves with data from two different source files, which contains one integer record per line):
#!/usr/bin/gnuplot
reset
set yrange[0:]
set title "Delta indexing time (in seconds)"
set ylabel "indexing time (seconds)"
set xlabel "indexer run number"
set grid
set style data linespoints
set terminal png size 1200,800
set output "/my/output/file.png"
set style data linespoints
plot "/my/data/source/file1" w lp title "Data 1 are here!", "/my/data/source/file2" w lp title "Data 2 are here!"