How to calculate gradient from semilogy plot graph?

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I have data and already plotted in semilogy graph,
I need to know the gradient from that graph
Any one can help me, please?
I use Matlab 2016b versiaon

Accepted Answer

Torsten on 14 Jul 2022
In the left part, your functional equation is approximately
f(x) = 10^(-3.2*x+70)
I don't know from your question whether you mean the gradient of your original function f(x) or that of log10(f(x)) that is shown in the plot.
Torsten on 15 Jul 2022
Edited: Torsten on 15 Jul 2022
I think I made a mistake in the calculation of your pressure function.
I assumed that in the semilogy plot, log(p) is plotted against t. But that's not true - it's also p that is shown.
The following code should be correct:
b = log10(70);
a = log10(6/70)/30;
syms t
p = 10^(a*t+b);
ans = 70.0000
ans = 6.0000
gradp = diff(p,t);
p = matlabFunction(p);
gradp = matlabFunction(gradp);
t = 0:0.1:40;

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