MATLAB doesn't let me plot 3 graphs in the same plot

I am using the hold on command but matlab still wont let me have 3 graphs in the same plot. I am solving ode with eulers step size 1 and 5 and also with ode45.

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h=1; %step size of 1
tm=(0:h:20);%t0=0 and tEnd=20
ve=zeros(size(tm)); %v modified in terms of t
ve(1)=500; %v0=500L
n=numel(ve); %amount of v values

for i=1:n-1
    f=10*exp(-tm(i)/20)-11;
    ve(i+1)=ve(i)+h*f; %applying Eulers method
end

htwo=5; %step size of 5
tn=(0:h:20);%t0=0 and tEnd=20
vi=zeros(size(tm)); %v modified in terms of t
vi(1)=500; %v0=500L
ne=numel(vi); %amount of v values

for i=1:ne-1
    f=10*exp(-tn(i)/20)-11;
    ve(i+1)=vi(i)+htwo*f; %applying Eulers method
end

tspan = [0 20]; %t0=0 and tEnd=20
v0 = 500; %initial volume
[t,v] = ode45(@(t,v) 10*exp(-t/20)-11, tspan, y0) %applying ode45

figure(1)
plot(t,v); grid on
hold on
plot(tm,ve(i+1),'green')
hold on
plot(tn,vi(i+1))
hold off
ylim([400 500])
xlim([0 20])
title(' Volume Change on Reservoir over 20 Days')
xlabel('time (days)')
ylabel('volume(L)')

Answer 1

[Guillaume]

Matlab plots exactly what you ask it.

plot(tm,ve(i+1),'green')

will plot the scalar value ve(21) (since after the loop i is 20) at each tm value as a single tiny green dot. Green is not the most visible colour, so you probably can't see it.

plot(tm, ve(i+1), 'g*')

would let you see the points better, but probably you meant to plot

plot(tm, ve)

I suggest you let matlab choose the colour as the defaults a lot better than 'green'. Also note that you only need one hold on. So your code could be:

plot(t, v);
hold on;
plot(tm, ve);
plot(tn, vi);

or you could everything with just one plot call and no need for hold on:

plot(t, v, tm, ve, tn, vi);

[Guillaume]

Note that if the above doesn't result in the expected plot, then you've made a mistake with your equations, it's nothing to do with the plot functions. As we don't know what your equations are meant to do, it's not really something we can help with at present.

Answer 2

[Fabio Freschi]

The problem is here

plot(tm,ve(i+1),'green')
...
plot(tn,vi(i+1))

ve(i+1) is a number, as well as vi(i+1), while tm and tn are vectors

[Camille Maradiaga Ponce]

Thanks you. How can I take all the answers for ve and vi and make them vectors in order to plot them?

Answer 3

[Fabio Freschi]

As Guillame pointed out, you just have to plot them (without the indexing):

h = figure;
hold on, grid on

plot(t,v);
plot(tm,ve);
plot(tn,vi);

ylim([400 500])
xlim([0 20])
title(' Volume Change on Reservoir over 20 Days')
xlabel('time (days)')
ylabel('volume(L)')

[Camille Maradiaga Ponce]

I dont know why the graph ends up looking like this

[Camille Maradiaga Ponce]

I dont know why the graph ends up looking like this

[Fabio Freschi]

It is the plot of your solution. I don't know the problem so I can't comment on the results.

[Guillaume]

Rather than postive massive (!) screenshots, export your figure as an image (png) and insert that.