Colour of legend doesn't change?

This is my plotting code

%%
%The Spectrum

figure(5);
S1 = (a.^2).*0.5;
plot(f,S1,'b');
hold on;
plot(f,S,'r');

legend ('Initial spectrum','JONSWAP Spectrum');
title('The Spectrum')
xlabel('Frequency (Hz)')
ylabel('Wave spectrum in (m^2/s)')

When I run it I got this one :

The colour of the legend is the same. How can I change it?

[KALYAN ACHARJYA]

Please share those undefined variables.

[Walter Roberson]

Is it possible that there is already something drawn in figure 5, and that "hold on" is set? After the figure(5) try cla()

[Muhammad Fakhruddin Abd Rahim]

what is cla?

[Muhammad Fakhruddin Abd Rahim]

clc
clear all
close all

myData = importdata('bathymetry.dat');
x = myData(:,1);
z = myData(:,2);

%Plot data
figure(1)
plot(x,z,'b') 
grid on
axis tight
xlabel ('Longitude {\circ}') 
ylabel ('Water depth (m)')
title('Bathymetry Profile')

%%
%Fully-developed sea
load('FDS.mat') %range from -40 to -2.4(the observer)
longitude = FDS(:,1); 
depth = -FDS(:,2);

figure(2);
plot(longitude,-depth);
grid on
axis tight
xlabel ('Longitude {\circ}') 
ylabel ('Water depth (m)')
title('Fully-Developed Sea Profile')

%Constant variable
gravity = 9.81; %m/s^2
EarthRadius = 6371000; %in m

% Define the frequency vector
Tmin = 3; 
Tmax = 30; 
nFreq = 30;
fmin = 1/Tmax; 
fmax = 1/Tmin;
f = linspace(fmin,fmax,nFreq);

% Define JONSWAP spectrum parameters
Hs  = 5;   % in m
Tp  = 20;  % in s
gam = 3;   % gamma factor is always greater than 1


%% 
% In this function, f=0 should be excluded
S = JONSWAP_spectrum(f,Hs,Tp,gam);
%
figure(3);
plot(f,S)
xlabel('Frequency (Hz)')
ylabel('Wave spectrum in (m^2/s)')
title('The JONSWAP Spectrum')
%set(findall(gcf, '-property', 'FontSize'), 'FontSize', 14)
%}

%% 
% Decomposition of the wave spectrum into regular waves
% The row of dt is 30
nWaves = numel(f); 
df = f(2) - f(1);
for i = 1:nWaves
    a(i,1) = sqrt(2*S(i)*df);
    omega(i,1) = 2*pi*f(i);
    k(i,1) = kfromw(omega(i,1), depth(1), gravity);
    cg(i,1) = 0.5*omega(i,1)/k(i); % check the 1/2 in the formula
    xg(i,1) = 0;    % Initial position
    dt(i,1) = 0;    % Initial position
end

%% 
% Evolution equations
% The column of dt is 2257
ndepth = size(FDS,1);
for it = 2:ndepth %(in hours)
    for i = 1:nWaves
        a(i,it) = a(i,1); % no dissipation / dispersion
        omega(i,it) = omega(i,1); % the frequency is not changed
        k(i,it) = kfromw(omega(i,it), depth(it),gravity); % Wave number associated with depth
        cg(i,it) = 0.5*omega(i,1)/k(i,it); % group velocity
        dtheta = (longitude(it)-longitude(it-1))/180*pi;
        dx = EarthRadius*dtheta;  
        dt(i,it) = dx/cg(i,it);
    end
end

Answer 1

[Walter Roberson]

Your a looks to have multiple columns, so S1 would have multiple columns.

plot(f,S1,'b');

would generate one line object per column.

[Muhammad Fakhruddin Abd Rahim]

S1 has 2257 column, my problem is the colour of the legend

[Walter Roberson]

If S1 has 2257 columns, then plot(f,S1) will produce 2257 line() objects. When you legend() providing only two texts, then by default legend() will pick up the first two of those, both of which are the ('b') line() objects, so both of them are given blue entries in the legend text.

You have some options:

1)

H1 = plot(f,S1,'b');
hold on;
H = plot(f,S,'r');
legend([H1(1), H(1)], 'Initial spectrum','JONSWAP Spectrum');

2) It is possible to merge all of the first drawing into a single line object by putting NaN between the segments to be drawn, such as

Stemp = S;
Stemp(end+1,:) = nan;

and then plotting Stemp(:) against a version of f that has been adjusted to have appropriate number of copies of the coordinates

3) Leave your existing graphics, but afterwards do

L = plot(nan, nan, 'b', nan, nan, 'r');
legend(L, 'Initial spectrum','JONSWAP Spectrum')

[Muhammad Fakhruddin Abd Rahim]

Thank you,I managed to plot it