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semilogy, loglog do not work in order to set the y axis on a logarithmic scale

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Yassine El Baaj
Yassine El Baaj on 12 Sep 2021
Edited: Jan on 12 Sep 2021
I am working on a numerical algorithm and I want to plot the error for 5 different shape parameters of my function. I want all the plots to figure on one same figure.
Here is the code that I use:
figure
hold on
for i=3:7
[coord_ctrs, errors] = ApproxAdap1D(i);
semilogy(coord_ctrs, (errors));
title("Senkung des RMS Fehlers");
grid;
end
Result:
As you can see, it does not work, the y axis does not have a logarithmic scale. loglog function displays tthe same result too.

Accepted Answer

Jan
Jan on 12 Sep 2021
Edited: Jan on 12 Sep 2021
After figure, hold on the YScale is determined already. Define it explicitly instead and you plot():
figure
axes('yscale', 'log');
hold on
title("Senkung des RMS Fehlers");
for i=3:7
plot(1:10, rand(1, 10));
end

More Answers (1)

Dave B
Dave B on 12 Sep 2021
When you use the log plotting functions they don't change the axis scale if hold is on. There's a note all the way at the bottom of the documentation pages (oddly, in algorithms). Here's the one from the semilogx page:
The semilogx function plots x-coordinates on a log scale by setting the XScale property of the axes to 'log'. However, if the axes hold state is 'on' before you call semilogx, the property does not change, and the x-coordinates might display on a linear scale.
Fortunately you can just set the relevant scale property:
hold on
for i = 1:3
plot(1:10,2*(rand(10,1)*i))
end
set(gca,'YScale','log')

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