MATLAB: Parabola plot not displayed correctly

parabola

I am trying to have a quadratic fit to my data.
X = randi(100,[40, 1])
Y = (2* (X.^2)) +(3*X) +(5);
X2 = [ones(size(X))  X  X.^2]
m3 = X2 \ Y;
y3 = X2*m3;
scatter(X,Y);
hold on
plot(X,y3)
ylabel('Y')
title('y= ax^2+bx+c', 'FontSize',18)
hold off;
untitled1.jpg
However, when I display the plot istead of a single parabolic fir I am getting multiple lines.
Please help. I experimented this while learning linear regression.

Best Answer

You are suing the random numbers for demo.....you have to sort them before you use: 
X = randi(100,[40, 1]) ;
X = sort(X) ; 
Y = (2* (X.^2)) +(3*X) +(5);
X2 = [ones(size(X))  X  X.^2]
m3 = X2 \ Y;
y3 = X2*m3;
scatter(X,Y);
hold on
plot(X,y3)
ylabel('Y')
title('y= ax^2+bx+c', 'FontSize',18)
hold off;
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