Files stuck in Gnuplot output -

i've been trying generate scripts delete previous versions of files before run actual code. example, within files there several set output "file1.txt" or "file2.txt" commands. after file has run, have unset output. when use main terminal , show ouput replies: output stdout i'd expect.

however, if try

    system `del "file1.txt"' 

the window closes without deleting file. opening shell , trying same command responds file being used process.

when try , delete file through explorer, says file in use 'wgnuplot'. i'm unsure how make second process let go of file!