How to make linear fit?

Hi; I want to plot a linear fit for the graphic in the attached file. And also I want to calculate slope of this linear fit? How can I do this? File is in the attached file.. Thanks a lot.

Answer 1

[Star Strider]

Try this:

I = openfig('test.fig');
Ax = gca;
L1 = findobj(Ax, 'Type','line');                                    % Find ‘line’ Object
x = L1.XData;                                                       % Extract X-Data
y = L1.YData;                                                       % Extract Y-Data
XL = [min(x) max(x)];                                               % Get X-Limits
P = [x(:), ones(size(x(:)))] \ y(:);                                % Estimate Linear Fit Parameters
LinFit = [XL(:) [1;1]] * P;                                         % Linear Fit
Slope = P(1);

hold on
plot(XL, LinFit, '-r', 'LineWidth',1.5)
hold off

[Steven Lord]

Or consider using the Basic Fitting UI directly in your figure.

[Matt Dickson]

Also consider using polyfit to perform the regression and simplify the code, if you want it done in the script.

[Star Strider]

My code estimates, calculates, and plots a linear fit.

It estimates the parameters as ‘P’, and then calculates a regression line in ‘LinFit’. This is likely more efficient than polyfit and polyval for a simple linear fit.

[nancy]

Hi,

I want to use polyfit command. But how can I write a code in the script to use polyfit?

[Star Strider]

To use polyfit:

I = openfig('test.fig');
L = findobj(gca, 'Type','line');                                   % Find ‘line’ Object
x = L(2).XData;                                                     % Extract X-Data
y = L(2).YData;                                                     % Extract Y-Data
x = x(y > -2);                                                      % Remove Outliers
y = y(y > -2);                                                      % Remove Outliers
XL = [min(x) max(x)];                                               % Get X-Limits
P = polyfit(x, y, 1);                                               % Estimate Linear Fit Parameters
LinFit = polyval(P, XL);                                            % Linear Fit
Slope = P(1);

figure
plot(x, y)
hold on
plot(XL, LinFit, '-r', 'LineWidth',1.5)
hold off

EDIT — Added plot image.

Answer 2

[Image Analyst]

See my attached polyfit() demo.

[nancy]

 When I execute the code; I have below message:

Polynomial is not unique; degree >= number of data points. > In polyfit at 71 In Untitled at 7

How can I solve it? Thanks a lot..

[Image Analyst]

The demo runs fine. How did you change it? Post your data and code.