I understand that you want to find an envelope for all the peak values of different curves. Looking at the image you shared, I assume each curve only has one peak value.
First, you can obtain the location and height of all your peaks and store them in an array.
For example:
[peakYCoords(1),peakXCoords(1)] = findpeaks(y,x);
Say all your peak values are in vector peakYCoords and their corresponding locations are in vector peakXCoords, you can use polyfit and polyval functions to fit a curve to these points, which would act as an envelope
Choose the value of n depending on the number of curves. See the documentation for polyfit for more details.
p = polyfit(peakXCoords, peakYCoords, n);
v = polyval(p, t);
plot(t, v)
Another alternative is to use the envelope function
