Post the data points, but the red line is clearly NOT the least-squares quadratic through the blue points; something is amiss.
If I use estimates of
>> den=[320 500 830]; dep=[0 -20 -110];
>> figure
>> plot(den,dep,'o')
>> b=polyfit(den,dep,2)
b =
-0.0003 0.1487 -15.1476
>> denfit=linspace(den(1),den(end));yhat=polyval(b,denfit);
>> hold on
>> plot(denfit,yhat,'r-')
>>
A quadratic will fit three points exactly.
To get a smooth curve, you have to evaluate at more than just the three points, but in your plot, the values aren't correct for those points.
Show your work so we can see where the mishap occurred...
