The parabola is "extreme" which is why you are seeing a "line". If you choose "x" values near the vertex it will look parabolic. The vertex is located close to x = 6. Try values around 6.
Note the value of y rises then falls.
how would you find this? Play with x values and watch the slope of the line. For some "x" values the line is increasing and others it is decreasing. You need to find roughly where it changes slope. in this case at x = 6 approximately.