Want to fit linear curve on my data.

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I have x, y data
x=[0;0.100000000000000;0.200000000000000;0.300000000000000;0.400000000000000;0.500000000000000;0.600000000000000];
y=[4.67178152947921e-06;4.67353333624452e-06;4.70560728038426e-06;4.74873086195845e-06;4.77333265701103e-06;4.84630647442201e-06;4.87015810633671e-06];
I want to plot x vs y and want to y-axis in log scale
plot(x,y)
set(gca,'YScale','log')
hold on
Note: x data starts from 0
Now I want to fit a line and show the slope of that curve fitting line + original curve
p=polyfit(x,(y),1);
q=polyval(p,x);
plot(x,q).
It seems to be not right because the fit line isn't straight ( it likes power fit or exponential) . Note log scale ( not log(data))
Please helps. Thanks
  2 Comments
Daniel Shub
Daniel Shub on 18 Mar 2013
Your x and y are not the same size ...

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Accepted Answer

Daniel Shub
Daniel Shub on 18 Mar 2013
Edited: Daniel Shub on 20 Mar 2013
I don't like working on log scales, I would rather take the log transform of my data
x=[0;0.100000000000000;0.200000000000000;0.300000000000000;0.400000000000000;0.500000000000000;0.600000000000000];
y=[4.67178152947921e-06;4.67353333624452e-06;4.70560728038426e-06;4.74873086195845e-06;4.77333265701103e-06;4.84630647442201e-06;4.87015810633671e-06];
xx = x;
yy = log(y);
plot(xx, yy, '*');
lsline;
p = polyfit(xx, yy,1);
text(max(xlim), max(ylim), ['Slope: ', num2str(p(1))], 'HorizontalAlignment', 'Right', 'VerticalAlignment', 'Top')
set(gca, 'YTickLabel', 10^6*exp(get(gca, 'YTick')))
  4 Comments
Daniel Shub
Daniel Shub on 20 Mar 2013
I got the output of polyfit wrong (I thought the first coefficient was the intercept). I changed the code. I also changed the scaling of the ticks. You can set them to be whatever you want.

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More Answers (1)

Jan
Jan on 19 Mar 2013
When you want a line in the logspace diagram, you need an exponential fit on the data. Or build the log of the data at first and fit the line afterwards.

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