In the calculator I typed in -x(x-1000/900). Should I have typed it differently? I also tried your way -x(x-1000)/900 and got a straight line. And even if I select zoomfit it would still show a straight line.
one more time ...
that's because you're looking at the parabola in a very small window. the parabola itself is very large.
any curve will look like a straight line if you look at it in a relatively small window. it's called the concept of local linearity.