Please, read the documentation of plotyy: Its first output argument is a

vector with handles to the two axes. You can use those to change properties

of the two axes separately:

x = 0:0.1:2*pi;

y1 = sin (x);

y2 = exp (x - 1);

ax = plotyy (x, y1, x - 1, y2);

set(ax(2))

set(ax(2), "ydir", "reverse");

--

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http://octave.1599824.n4.nabble.com/Octave-General-f1599825.html