The default terminal is normally X11_SingleWindow, except when PyXPlot is used non-interactively – i.e. one or more command scripts are specified on the command line, and PyXPlot exits as soon as it finishes executing them. In this case, the X11_SingleWindow would not be a very sensible terminal to use as any plot window would close as soon as PyXPlot exited. The default terminal in this case changes to eps.
This rule does not apply when the special ‘–’ filename is specified in a list of command scripts on the command line, where an interactive terminal will operate between running a series of scripts. In this case, PyXPlot detects that the session will be interactive, and defaults to the usual X11_SingleWindow terminal. Conversely, on machines where the DISPLAY environment variable is not set, PyXPlot detects that it has access to no X-terminal on which to display plots, and defaults to the eps terminal.