How to linearly fit a semilog graph

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Sonia Goyal 2021-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/749924-how-to-linearly-fit-a-semilog-graph

评论： Star Strider 2021-3-4

Data.xlsx

Hello

I have my data file which is attached below. I have to plot this as semilog ( log scale on Y-axis) and have to linearly fit like the image attached. Please help me with the same

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Answer 1

Jonas 2021-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/749924-how-to-linearly-fit-a-semilog-graph#answer_627609

https://nl.mathworks.com/matlabcentral/answers/163349-linear-fit-of-a-semilog-graph?s_tid=answers_rc1-1_p1_MLT

https://nl.mathworks.com/matlabcentral/answers/78694-linear-regression-on-a-semi-log-scale?s_tid=answers_rc1-3_p3_MLT

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Sonia Goyal 2021-2-26

Thank you for your rresponse. But this isn't working for this data.

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Answer 2

Star Strider 2021-2-26

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https://ww2.mathworks.cn/matlabcentral/answers/749924-how-to-linearly-fit-a-semilog-graph#answer_633939

在 MATLAB Online 中打开

Try this:

D1 = readmatrix('Data.xlsx');
x = D1(:,1);
y = D1(:,2);
[ymax,idx] = max(y);
idxrng = idx:numel(y);
objfcn = @(b,x,minlim) b(1).*exp(b(2).*x) + minlim;
B = fminsearch(@(b) norm(y(idxrng) - objfcn(b,x(idxrng),y(end))), [ymax; rand]);
figure
plot(x, y)
hold on
plot(x(idxrng), objfcn(B,x(idxrng),y(end)), '-r', 'LineWidth',1.5)
hold off
grid
set(gca, 'YScale','log')

It’s not easy to fit those data.

Using:

idxrng = 1:numel(y);
objfcn = @(b,x,minlim) b(1).*x.*exp(b(2).*x) + minlim;

instead, might be more accurate.

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Star Strider 2021-3-1

在 MATLAB Online 中打开

I’m not certain what you want. Note that the ‘11 to 40’ constraint was not previosly mentioned.

An exponential fit is appropriate here because it deals realistically with the additive noise. Doing a linear fit on logarithmically-transformed data is not appropriate, since it violates a number of assumptions of the least-squares method. (I will not post code that I know to be in error.)

Try this:

D1 = readmatrix('Data.xlsx');
xr = D1(:,1);
yr = D1(:,2);
idxrng = (xr >= 11) & (xr <= 40);                                   % Use Only 11 - 40 sec
x = xr(idxrng);                                                     % Use Only 11 - 40 sec
y = yr(idxrng);                                                     % Use Only 11 - 40 sec
[ymax,idx] = max(y);
objfcn = @(b,x) b(1).*x.*exp(b(2).*x) + b(3);
B = fminsearch(@(b) norm(y - objfcn(b,x)), [ymax; rand; y(end)]);
figure
plot(x, y)
hold on
plot(x, objfcn(B,x), '-r', 'LineWidth',1.5)
hold off
grid
set(gca, 'YScale','log')
text(25, 0.0015, sprintf('slope = $%9.6f$\nintercept = $%9.6f$', B([2 1])), 'Interpreter','latex')

.

Sonia Goyal 2021-3-4

Thank you for you response.

Star Strider 2021-3-4

My pleasure.

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How to linearly fit a semilog graph

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