Fitting a straight line to a specific region of the graph

Hello, I want to fit a polynomial of 1 degree to a graph. The linear function should only be fittet to a certain region which I determined with T. Somehow it won't work and I can't figure out why.

%Choosing section for the linear fit (preperiod)
T=(0:67.842);
xpreperiod= xm(T);
ypreperiod= ym(T);
%erforming a linear fit using polynomial of 1 degree 
%p beeing the coefficients and S beeing the error
[p, S]=polyfit(xpreperiod,ypreperiod,1);
fitT=polyval(p,xpreperiod)
plot(xm,ym,'b') % plotting original data in blue
plot(xpreperiod,fitT,'r') % plotting to fitted line in red

Answer 1

[Star Strider]

I suspect the problem is in using‘T’ as a subscript, since subscript values must be integers greater than 0 or logical values.

Assuming that it is a time vector, perhaps —

T=(0:67.842);
Tv = T(T>=0 & T<=67.842);                   % Create 'logical' Index Vector
xpreperiod= xm(Tv);
ypreperiod= ym(Tv);

It would be nice to have your data to test this with.

.

[Luna Häring]

Thank you. I tried to do that but it still won't work. With the code in my question a poly1 fit is performed, but this is only as long as the range I have chosen MATLAB to fit to. I want the linear fit to go to the end of the data.

[Star Strider]

Try this —

T1 = readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1057565/Mehl.txt', 'VariableNamingRule','preserve')

T1 = 2359×2 table
    Var1      Var2 
    _____    ______

        0    23.813
    0.254    23.813
    0.511    23.811
    0.769    23.811
    1.028    23.821
    1.281    23.821
    1.539    23.821
    1.797     23.81
     2.05     23.81
    2.303     23.81
    2.562    23.814
     2.82    23.814
    3.073    23.825
    3.331    23.825
    3.589    23.825
    3.845    23.822

xm = T1{:,1};
ym = T1{:,2};

%Choosing section for the linear fit (preperiod)
T=(0:67.842);
Tv = (xm>=0 & xm<=67.842);                % Create 'logical' Index Vector
xpreperiod= xm(Tv);
ypreperiod= ym(Tv);

%erforming a linear fit using polynomial of 1 degree 
%p beeing the coefficients and S beeing the error
[p, S]=polyfit(xpreperiod,ypreperiod,1);
fitT=polyval(p,xm);

figure
plot(xm,ym,'b') % plotting original data in blue
hold on
plot(xm,fitT,'r') % plotting to fitted line in red
hold off

.

[Luna Häring]

Thank you so much, it worked!!!

[Star Strider]

My pleasure!

If my Answer helped you solve your problem, please Accept it!

.

Answer 2

[Image Analyst]

Try this:

data = readmatrix('Mehl.txt');
T = data(:, 1);
ym = data(:, 2);
plot(T, ym, 'b-', 'LineWidth', 2);
grid on;
indexes = T > 0 & T < 67.842;
% Do the fit
coefficients = polyfit(T(indexes), ym(indexes), 1);
% Draw the fitted line.
xFit = T(indexes); % Or T if you just want to see the line over the whole plot.
yFit = polyval(coefficients, xFit);
hold on;
plot(xFit, yFit, 'r-', 'LineWidth', 2)
fontSize = 20;
xlabel('T', 'FontSize', fontSize)
ylabel('y', 'FontSize', fontSize)
caption = sprintf('y = %f * T + %f', coefficients(1), coefficients(2));
title(caption, 'FontSize', fontSize)