set obs 10000

gen x=rnormal(0,1)

gen y=30*x +20*x^2+100*rnormal(0,1)

* put x on an ordinal scale, that runs from -5 to 5

egen xx = cut(x), group(11)
replace xx=xx-5

reg y c.xx##c.xx

margins, at(xx=(-5/5))
marginsplot