Bi-linear (piecewise) curve fitting

Can someone explain how to plot the two linear lines (Billinear curve fiiting) from the existing curve?

NOTE:-

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hello

try this (a fairly simple code)

T1 = readtable('testdata.xlsx');
x = T1.Displacement;
y = T1.BaseForce;

% top flat curve (apex)
[m,idy] = max(y);
ym = m*ones(size(y));
plot(x,y,'*-',x,ym,'g','Linewidth',3);
hold on
% red curve ( "yellow" net area must be zero)
% origin = 0,0 (first data point)
x1 = x(1);
y1 = y(1);
% let's find out which second point of the curve let us find the min area
xx = x(1:idy);
yy = y(1:idy);
for ck = 2:idy
    x2 = x(ck);
    y2 = y(ck);
    slope = (y2-y1)/(x2-x1);
    yl = y1 + slope*(xx - x1);
    % compute yellow area A
    A(ck) = trapz(xx,(yy-yl));
    
end
 
% find zero crossing value for A (with linear interpolation)
threshold = 0; 
xx_zc = find_zc(xx,A,threshold);
yy_zc = interp1(xx,yy,xx_zc);
slope = (yy_zc-y1)/(xx_zc-x1);
yl = y1 + slope*(xx - x1);
plot(xx,yl,'r','Linewidth',3);

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Bi-linear (piecewise) curve fitting

Bi-linear (piecewise) curve fitting