The following examples show how to add the second Y axis that ranges from 20 to 25:
1. Add the second Y axis manually:
% Plot function.
h = plot(1:10)
hold on
box off
% Create second Y axes on the right.
a2 = axes('YAxisLocation', 'Right')
% Hide second plot.
set(a2, 'color', 'none')
set(a2, 'XTick', [])
% Set scala for second Y.
set(a2, 'YLim', [20 25])
2. Use PLOTYY:
figure;
[a,h1,h2]=plotyy(1:10, 1:10, 1:10, linspace(20,25,10));
delete(h2);
set(a(2),'ycolor', get(a(1), 'ycolor'));
