By default, when graphical output is sent to a file – i.e. a postscript file or a bitmap image – pre-existing files are overwritten if their filenames match 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.