Flipping y-axis of findpeaks-plot

Hi!

To find minima-values of an array I multiplied my array with -1 and used findpeaks. To graphically show the minimas I want to reverse the findpeaks-plot but can't figure out how. Any suggestions?

E_bat3 = E_bat2.*(-1);

findpeaks(E_bat3(winter_hours)) %Graph I want to flip.

Thanks!

Answer 1

[Star Strider]

You may be making this too difficult.

Example —

a = linspace(0, 4*pi);
y = sin(a);

[pks,pkloc] = findpeaks( y,a);                          % Maxima
[vls,vlloc] = findpeaks(-y,a);                          % Minima

figure(1)
plot(a, y, '-b')
hold on
plot(pkloc, pks, '^r', 'MarkerFaceColor','r')
plot(vlloc, -vls, 'vg', 'MarkerFaceColor','g')
hold off
grid
axis([xlim    -1.1  1.1])
legend('Data', 'Peaks', 'Valleys')

Note that to plot the valleys (‘vls’) or minima correctly, simply negate them in the plot call. So to find them, use:

[vls,vlloc] = findpeaks(-y,a);                          % Minima

and to plot them or use them in other calculations correctly, negate them again to restore their correct values:

plot(vlloc, -vls, 'vg', 'MarkerFaceColor','g')

Answer 2

[Adam]

Don't you just want to negate the peak amplitude results rather than flip?

[Nils Norlander]

Yes, the peaks are now found as maximas at negative values - and I want to show them as minimas at positive values.

[Adam]

But if they are minima then they will be larger as an absolute value than those around them so how can you display them still as minima if you plot the result as positive only?

I guess if you really want to do that then you need to subtract from whatever the largest absolute value in the results is.

Answer 3

[Greg Dionne]

I think Star Strider's answer is the right approach, but if you prefer the graphical 'look and feel' of the FINDPEAKS plot, then try this:

load mtlb
findpeaks(mtlb,Fs,'MinPeakProminence',1);
figure
findpeaks(-mtlb,Fs,'MinPeakProminence',1);
set(gca,'YDir','reverse');
set(findobj(gca,'Tag','Peak'),'Marker','^');
set(gca,'YTickLabel',cellfun(@(x)num2str(-str2num(x)),get(gca,'YTickLabel'),'UniformOutput',false))

[Nils Norlander]

Thank you so much! Think I've almost got it figured out but I get the error: "Expected Y to be a vector" in Findpeaks.

Should I just replace "mtlb" with my Y-values and Fs with my X-values?

Once again, thank you for taking your time to help me!

[Greg Dionne]

Yes, that should work. FINDPEAKS expects vector input for Y. So in your case you could do findpeaks(-y, a) just like you did before, but then do the graphics commands above. I think they'll work no matter what input you have (so long as you actually have peaks). Otherwise, finding the 'peak' tag will fail because no peaks will actually be plotted.