It is definitely close to linear in the region you are plotting it.
To plot it and see its quadratic shape, find the inflection point and use that as the centre of the plot range.
Try this —
syms x
y=(-53.68/(2*230))*((x).^2)+500.0089*x+55
y_inflpt = vpasolve(diff(y)==0)
figure
fplot(y,[-55 55]+double(y_inflpt))
grid
.
