Plot tangent line on part of the curve

Hello all,

I am trying to plot tangent line but only on part of the curve not all, like first 25% or less on the curve.....

Also would like the tangent to start from zero

Then add another tangent a bit on right side...

I attached both file and matlab code, but I could not do it...

Looking to hearing from anyone

clearvars
clc
close all; 

data            =   xlsread('data_tangent.xlsx');
x               =   data(:,1);                        
y               =   data(:,2); 
x_n             =   x-x(1);
y_n             =   y-y(1);


B             =       x_n\y_n;                     % Force Origin Intercept

x1            =       [0;x_n];
y1            =       B*x1;

fig=figure;
hold on
plot(x_n,y_n)
plot(x1,y1)

Thank you,

Answer 1

[Star Strider]

Try this —

Data = readmatrix('data_tangent.xlsx')

Data = 13042×2

1.9803 7.3700 1.9823 7.3500 1.9853 7.3800 1.9873 7.4000 1.9903 7.3100 1.9933 7.5100 1.9953 7.5300 1.9973 7.4300 2.0003 7.5100 2.0022 7.7400

x = Data(:,1);
y = Data(:,2);

[ymax,idx] = max(y);

Binit = x(1:5) \ y(1:5)

Binit = 3.7086

Bymax = x(idx) \ y(idx)

Bymax = 3.2160

Line_init = x*Binit;
Line_initv = Line_init <= ymax;

Line_ymax = x*Bymax;
Line_ymaxv = Line_ymax <= ymax;

figure
plot(x, y)
hold on
plot([0;x(Line_initv)], [0;Line_init(Line_initv)], '-r')
plot([0;x(Line_ymaxv)], [0;Line_ymax(Line_ymaxv)], '-r')
% plot(x(idx),y(idx),'r+')
hold off
grid
axis('equal')
xlabel('X')
ylabel('Y')
axis([0 max(x)  0 max(y)])

.

[Mark Sc]

@Star Strider seems working but may i ask what are the indices of 1:5?

as when I apply it to other data seems not working

Let me know ??

Thanks anyway

[Star Strider]

My pleasure!

Indices 1:5 are simply the initial part of the curve that is used to calculate the slope constant. They can be anything you want to use to define the initial part of the curve.

My code was designed for the data provided. To use it with other data, I need to see the other data and to know what the tangent lines for it are supposed to be, or at least more generally what the second tangent line criteria are.