Hello!
This should do it:
%% Import data
x = xlsread('Spectrum.xlsx','Sheet1','A1:A2503');
y = xlsread('Spectrum.xlsx','Sheet1','B1:B2503');
%% Plot data
plot(x,y)
hold on;
%% Find peaks and dips in the signal
% Peaks
[pks, pks_locs] = findpeaks(y,x); % Finds peaks and their locations
plot(pks_locs, pks, '--k') % Plots a black dashed-line connecting peaks
% Dips
[dip, dip_locs] = findpeaks(-y, x); % Find dips and their locations
plot(dip_locs, -dip, '--k') % Plots a black dashed-line connecting dips
hold off;
Cheers,
Manny
