MATLAB: Can someone help me with enveloping the curve

dataenvelope

I need to get the result as attached. I have included the data required to plot the curve.

Have a look on envelope.

S = importdata('data.txt') ;
data = S.data ;
t = data(:,1) ; y = data(:,2) ;
plot(t,y) ;
hold on
[up,lo] = envelope(y);
plot(t,up,'r')

Your data is in text file data.txt

data = importdata("1_ 1_ 1.txt") ; 
plot(data)

If you have Curve Fitting Toolbox, you can generate everything you need as follows

X = 1:100;
X = X';
Y = X.^2 + 3*X + 5 + randn(100,1);
[foo Gof] = fit(X,Y, 'poly2')
scatter(X,Y)
hold on
plot(foo)