Parabola plot not displayed correctly

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;

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.

Answer 1

[KSSV]

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;

[Sparkle89]

Thank you.It worked. Can you explain the need for sorting because I could fit a line without sorting?