Line plot on top of a log log plot
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I have a loglog plot of data in MATLAB and I am trying to overlay a linear fit. The linear data fitting tool doesn't work, so I was hoping to simply plot another line of slope -11/3. This line should be linear in a loglog plot, but I am having trouble plotting. Can anyone help? New to MATLAB so any advice would be useful. I have attached a picture of the plot I keep getting. The blue line is the loglog data plot, and the orange line is MATLAB's "linear fit" when using the basic plotting tool.
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David Goodmanson
2017-7-11
编辑：Walter Roberson
2017-7-12
Hello Cameron, If you have a linear fit in the logs,
log(y) = (-11/3)log(x) + A
then exponentiate both sides:
y = x^(-11/3) exp(A)
and with C = exp(A), the fitting line (linear on loglog plot) is
y = C*x.^(-11/3)
where you just have to provide C. C could come from the same theory that gives the -(11/3), in which case you have a fit with no adjustable parameters, nice if it works. Or you could calculate C by using one (x,y) data point, or you could fit A from the first equation by taking the difference between log(y) and (-11/3)*log(x), and averaging that difference over all the values to get A. Then C = exp(A).
If you prefer log base 10, then replace log by log10 and C = exp(A) by C = 10^A everywhere.
6 个评论
Cameron Park
2017-7-11
Walter Roberson
2017-7-12
hold on
?
David Goodmanson
2017-7-12
编辑：David Goodmanson
2017-7-12
You could do independent plot commands with 'hold on' active like Walter has mentioned, or you could do
y1 = C*x.^(-11/3);
loglog(x,y,x,y1) [corrected per Walter's comment]
Walter Roberson
2017-7-12
Well, loglog(x,y,x,y1)
David Goodmanson
2017-7-12
yes.
Cameron Park
2017-7-12
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