plot interface undefined

I compiled ISIS for a PPC OS/X G5 running Leopard (10.5.1).  If I try to plot to
a window that has not been opened with open_plot(), then the plot routines seem
to get permanently jammed... I can't open a plot after that.  (See below.)

I haven't seen this problem on other platforms...

Thanks,
Rob

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Welcome to ISIS Version 1.4.9
Copyright (C) 1998-2007 Massachusetts Institute of Technology

isis> open_plot("/xwin");
1
isis> plot_quit;
isis> plot(0, 1);
Error: plot interface is undefined
Failed: initializing plot window
plot failed
Integer_Type[1]
0
2147483647
1
0
isis> open_plot("/xwin");
Error: plot interface is undefined
Failed: initializing plot window
-1
isis> _isis_version;
10409

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