Octave: Use 'qt' graphics_toolkit() if available, 'gnuplot' else

This code uses the "qt" graphics toolkit only if available and defaults to using "gnuplot" else:

gts = available_graphics_toolkits()
qt_available = any(strcmp(gts, 'qt'))
if qt_available
  graphics_toolkit("qt")
else
  graphics_toolkit("gnuplot")
endif

See Octave: Check if cell array of strings contains string for details on how to check if a string like "qt" is in a cell array of strings like the one returned by available_graphics_toolkits().