adding trendline to a plot

Good Day,

Please how do I add trendline(s) to a certain straight line portion(s) of a plot and how to extend the trendline to touch y and or x axis and the y and or x axis value determined?

Another related question is that how do I insert horizontal line on a plot and extend it to touch y axis and the y axis value determined?

Thanks

Best regards,

Isa

[Alex Backer]

See https://www.mathworks.com/matlabcentral/fileexchange/76467-trendline

Answer 1

[Image Analyst]

I'd fit the portion of the data you're interested in to a line using polyfit. Then use polyval to get the points on the line over the entire range that you're interested in. See this demo:

clc;    % Clear the command window.
workspace;  % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 20;

% Define equation.
x = -2:20;
y = (x-2).^2;

% Plot it
plot(x,y, 'bo-', 'LineWidth', 3);
grid on;
title('y = (x-2).^2', 'FontSize', fontSize);
xlabel('X', 'FontSize', fontSize);
ylabel('Y', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Give a name to the title bar.
set(gcf,'name','Demo by ImageAnalyst','numbertitle','off') 

% Fit the range fro 10 to 20 with a line.
limitedRange = 10:20;
coeffs = polyfit(x(limitedRange), y(limitedRange), 1)
% Define the range where we want the line
xFitting = 0:19; % Or wherever...
yFitted = polyval(coeffs, xFitting);

% Plot the fitted line over the specified range.
hold on; % Don't blow away prior plot
plot(xFitting, yFitted, 'ro-', 'LineWidth', 2);
legend('Original Data', 'Line Fit');

[Image Analyst]

Isa, did that work for you?

[Isa Isa]

I am still having problem with it. I tried using your code but Trendline was shown in the legend but not on the plot where I want it. My data as below x=[ 0 0.2762 0.3488 0.4052 0.5242 0.5806 0.6129 0.6431 0.6673 0.6976 0.7238 0.7520 0.7944 0.8448]; y=[ 0 0.0230 0.0370 0.0480 0.0760 0.0900 0.1000 0.1180 0.1380 0.1600 0.1860 0.2130 0.2880 0.4010]; I want the trendline at y range between y(9) to y(14).

Thanks Isa

[Image Analyst]

I just put your data into my demo and got this. Is this what you want?

clc;    % Clear the command window.
workspace;  % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 20;

% Define equation.
x=[ 0 0.2762 0.3488 0.4052 0.5242 0.5806 0.6129 0.6431 0.6673 0.6976 0.7238 0.7520 0.7944 0.8448]; 
y=[ 0 0.0230 0.0370 0.0480 0.0760 0.0900 0.1000 0.1180 0.1380 0.1600 0.1860 0.2130 0.2880 0.4010];

% Plot it
plot(x,y, 'bo-', 'LineWidth', 3);
grid on;
title('y = (x-2).^2', 'FontSize', fontSize);
xlabel('X', 'FontSize', fontSize);
ylabel('Y', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Give a name to the title bar.
set(gcf,'name','Demo by ImageAnalyst','numbertitle','off') 

% Fit the range from index 1 to 14 with a line.
limitedRange = 1:14;
coeffs = polyfit(x(limitedRange), y(limitedRange), 1)
% Define the range where we want the line
xFitting = 9:14; % Or wherever...
yFitted = polyval(coeffs, xFitting);

% Plot the fitted line over the specified range.
hold on; % Don't blow away prior plot
plot(xFitting, yFitted, 'ro-', 'LineWidth', 2);
legend('Original Data', 'Line Fit');

[Isa Isa]

I run the code and see that trendline/line fit is not passing through any of the y data. Like your original demo, I want the trendline to fit and pass through certain region of the curve. It may be early region, middle region or late region of the curve.

Thanks Isa

[Image Analyst]

It was not clear when you said " I want the trendline at y range between y(9) to y(14)." because neither your x values nor your y values go through the range 9-14. See the line that says:

xFitting = 9:14; % Or wherever...

"whatever" means that you are supposed to change it to whatever you want for the x values. For example if you want 50 fitted (estimated) values between x(1) and x(14), you'd change that line to

xFitting = linspace(x(1), x(14), 50);

or if you wanted to make a point every 0.01 from x(1) to x(14), you'd do this:

xFitting = x(1) : 0.01 : x(14);

[Isa Isa]

Thanks. I have got it. I really appreciate your assistance. Another issue is that if I click on any point on my original data curve and fit curve, I want to see the x and y data for that point I click on the curve. Please assist.

Thanks Isa

[Image Analyst]

Can't you use ginput() to determine which actual data point is closest to the point you clicked at? That's what I'd try. If that didn't work I'd search Answers - I think that has been asked before.

If your original question is solved, go ahead and mark it as Answered.

[Isa Isa]

Thanks. ginput works well.

[Isa Isa]

Hi, Please if I want the slope of the 'Line Fit' how do I get it?

Thanks

Regards

[José-Luis]

 doc polyfit

[Isa Isa]

Hi, Please assist on this. I try to add trendline to a semilogx plot but didn't succeed. Please check the code below.

% Define equation. x=[ 90868 68151 45434 34076 27261 13631 6816 3408 2273 1948 1705 1137 853 683 569 455 342 274 228 190]; y=[ 3680 3723 3800 3866 3920 4103 4250 4320 4340 4344 4350 4364 4373 4379 4384 4393 4398 4402 4405 4407];

% Plot it semilogx(x,y, 'bo-', 'LineWidth', 3); grid on; % Enlarge figure to full screen. set(gcf, 'units','normalized','outerposition',[0 0 1 1]); % Give a name to the title bar. set(gcf,'name','Demo by ImageAnalyst','numbertitle','off')

% Fit the y data range with a line (limitedRange). limitedRange = 17:20; coeffs = polyfit(x(limitedRange), y(limitedRange), 1); xFitting = linspace(200, 90000, 50); yFitted = polyval(coeffs, xFitting);

% Plot the fitted line over the specified range. hold on; plot(xFitting, yFitted, 'ro-', 'LineWidth', 2); legend('Original Data', 'Line Fit');

Thanks Isa

[Ananya Roy]

Hi Isa,

I have similar problem as you had. Please let me know if you could solve that problem and how.