9.4 Backing up over-written files
By default, when graphical output is sent to a file – i.e. a PostScript file or a bitmap image – any pre-existing file is overwritten if its filename matches that of the file which Pyxplot generates. This behaviour may be changed with the set backup command, which has the syntax:
set backup set nobackup
When this switch is turned on, pre-existing files will be renamed with a tilde () appended to their filenames, rather than being overwritten.