# adding trendline to a plot

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Isa Isa am 23 Dez. 2012
Kommentiert: Alex Backer am 1 Jun. 2020
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
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Alex Backer am 1 Jun. 2020

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### Akzeptierte Antwort

Image Analyst am 23 Dez. 2012
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');
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Isa Isa am 6 Jan. 2013
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 am 20 Feb. 2017
Hi Isa,
I have similar problem as you had. Please let me know if you could solve that problem and how.

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