problem with gnuplot and octave, 2.9.19, MSVC build on windows XP

[N. B.]

Hello,
I need help with two things:
1) How can I change the graphics backend back and forth without quitting octave?
2) Currently, when I set the default backend to gnuplot, I get an error: 
(I copy and paste from cmd.exe window:)
error: drawnow: failed to open connection to gnuplot
error: evaluating if command near line 117, column 3
error: called from `drawnow:open_gnuplot_stream' in file `E:\Programs\Octave\share\octave\2.9.19\m\plot\drawnow.m'
error: evaluating if command near line 78, column 8
error: evaluating if command near line 77, column 6
error: evaluating if command near line 74, column 4
error: evaluating if command near line 72, column 2
error: evaluating for command near line 71, column 7
error: evaluating if command near line 45, column 5
error: called from `drawnow' in file `E:\Programs\Octave\share\octave\2.9.19\m\plot\drawnow.m'

However, if I set the backend to "jhandles" (in a startup file deep in the octave file hierarchy), it runs fine.
Probably you won't need it, but a very simple 3-line script is attached that demonstrates the problem, provided that it's not a mysterious unreproducible issue on other machines.
Kind regards,

Nusret


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