load https://www.mathworks.com/matlabcentral/answers/uploaded_files/1115780/y2_max.mat
I dunno whassup w/ this last couple days -- too much grief download that many files; please combine them all into one .mat file and also use the -v7.3 switch so folks can read it here.
That aside, a couple suggestions of what you could do although your data are such that the quadratic fits very well so the overlaying of the fit versus the data is going to be present no matter what you do.
One alternative presentation might be
plot(y2_max,St20_1,'x') % plot the data itself as points only, no line
hold on
plot(y2_max,trend_parabolic) % then add the fitted line
yyaxis right % and subsequently, add the residual on RH axis
resid=max,St20_1-trend_parabolic; % residuals/differences
plot(y2_max,resid) % then add the fitted line
Salt to suit...
