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Colour of legend doesn't change?

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This is my plotting code
%The Spectrum
S1 = (a.^2).*0.5;
hold on;
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 :
figure 5.PNG
The colour of the legend is the same. How can I change it?


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Walter Roberson
Walter Roberson on 14 Nov 2019
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
clear all
close all
myData = importdata('bathymetry.dat');
x = myData(:,1);
z = myData(:,2);
%Plot data
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);
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);
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
% 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);

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Accepted Answer

Walter Roberson
Walter Roberson on 14 Nov 2019
Your a looks to have multiple columns, so S1 would have multiple columns.
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
Walter Roberson on 14 Nov 2019
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:
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

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