fplot with two y-axis

I would like to fplot two curves Sp and St. Sp in the left Y-axis and the ST in the right Y-Axis. I can get the figure with two axis, I can plot the two curves independently but not together in a single graph. Is it possible?

I am using this code

figure (10)
hold on
fplot(Sp,[0.001 1000],"black")
yyaxis left
ylabel("S_{p}")
ylim ([0.000 0.04])
hold on
fplot(St,[0.001 1000],"--")
yyaxis right
ylabel("S_{t} (JK^{-1})")
ylim ([0.000 0.004])
xscale log
box on
xlabel("R_{2}/R_{1}")
hold off

Thank you very much

Answer 1

[Star Strider]

This seems to work —

Sp = @(t) (5+sin(2*pi*log(t)))/750;
St = @(t) (1+cos(1.5*pi*log(t)))/500;

figure (10)
yyaxis left
fplot(Sp,[0.001 1000],"black")
ylabel("S_{p}")
ylim ([0.000 0.04])
yyaxis right
fplot(St,[0.001 1000],"--")
ylabel("S_{t} (JK^{-1})")
ylim ([0.000 0.004])
xscale log
box on
xlabel("R_{2}/R_{1}")
hold off

.

Answer 2

[Shashi Kiran]

Hi @Angel Cuadras,

I understand that you want to plot two curves using yyaxis with fplot.

Based on the code you provided, here are some corrections made to achieve the correct result.

% Example functions
Sp = @(x) 0.02 * sin(log(x)) + 0.02; 
St = @(x) 0.002 * cos(log(x)) + 0.002; 

figure(10)
hold on

% Plot Sp using the left y-axis
yyaxis left
fplot(Sp, [0.001 1000], 'black')
ylabel("S_{p}")
ylim([0.000 0.04])

% Plot St using the right y-axis
yyaxis right
fplot(St, [0.001 1000], '--')
ylabel("S_{t} (JK^{-1})")
ylim([0.000 0.004])

% Common settings
set(gca, 'XScale', 'log') % Set x-axis to log scale
box on
xlabel("R_{2}/R_{1}")

hold off

Refer to the following documentations for more details about the functions:

1. yyaxis: https://www.mathworks.com/help/matlab/ref/yyaxis.html?s_tid=doc_ta
2. set: https://www.mathworks.com/help/matlab/ref/set.html?s_tid=doc_ta

Answer 3

[dpb]

You forgot to define the two variables to plot, but presuming they were defined, then

yyaxis left
fplot(Sp,[0.001 1000],"black")
ylabel("S_{p}")
ylim ([0.000 0.04])
yyaxis right
fplot(St,[0.001 1000],"--")
ylabel("S_{t} (JK^{-1})")
ylim ([0.000 0.004])
xscale log
xlabel("R_{2}/R_{1}")

should work. You tried to set the axes targets after the fact, not before...