Why does my sin graph look like a triangle?

[Cris LaPierre]

We would have to know what your function dfdt is doing in order to say for certain. Please copy/paste all your code here so we can run it and see.

[James Tursa]

@Roos Never post code as an image ... we can't run images at our end. Please always post code as regular text highlighted with the CODE button.

[Roos]

@Cris LaPierre @James Tursa , here you go, sorry I did not realize we could not send photos

function za = dfdt (z,fs)
% za = dfdt (z,fs)
% determines numerically the derivative za 
% of signal z.
% z is registered at sampling frequency fs.
h = 1/fs;
n =length (z);
for i = 2:n-1
    za (i) = (z(i+1) - z(i-1))/(2*h);
end
za(1)= (z(2)-z(1))/h;
za(n)= (z(n)-z(n-1))/h;

[Cris LaPierre]

You can obviously post pictures, but then you are asking the community to rewrite yout code from a picture. It's much simpler for us if you just copy/paste your code instead.

Answer 1

[Cris LaPierre]

Your sin wave is plotted just fine. However, it looks like a straight line because the magnitude of your second plot (in red) is so much larger (1 vs 5e5)

[Roos]

how do I change the magnitude?

[Stephen23]

"how do I change the magnitude?"

Rather than changing your data, I suspect that YYAXIS or PLOTYY is what you are looking for:

https://www.mathworks.com/help/matlab/ref/yyaxis.html

https://www.mathworks.com/help/matlab/ref/plotyy.html

[Cris LaPierre]

BTW, the derivative of sin is cosine. So the correct plot should show a sin and cosine wave of the same frequency and amplitude.