How can I superimpose data on a spectrogram

How can i superimpose the line of the SO power signal on to the powerspectrum ?

This is how i tried it:

hold on 

imagesc(log(flip(PXX_a{1,1})))

space=linspace(1,size(sleepstage{1,1},1),5);
set(gca,'xtick',[space],'xticklabel',[0:5])

xlabel('Time','FontSize',20)
yyaxis left 
ylabel('Powerspectrum','FontSize',20)
set(gca,'fontsize',20);

plot(pow2db((PXX_a{1,1}(1,:))))
yyaxis right
ylabel('SO power','FontSize', 20)

hold off