MATLAB polyfit, palyval not working correctly

Basically I'm just trying to curve fit some data but the line of best fit is not correct. Here is the code I have so far

clear all; close all
c = xlsread('Elastic Modulus.xlsx','Sheet1');
temp = c(:,1);
carbonSteel = c(:,2);
plot(temp,carbonSteel,'*')
hold on
p = polyfit(temp,carbonSteel,3);
y = polyval(p,carbonSteel);
plot(temp,y)

The temp and carbonsteel columns contain the values:

temp =       
  -200
  -129
   -73
    21
    93
   149
   204
   260
   316
   371
   427
   482
   538
   593
   
carbonSteel =
   31.4000
   30.8000
   30.2000
   29.5000
   28.8000
   28.3000
   27.7000
   27.3000
   26.7000
   25.5000
   24.2000
   22.4000
   20.4000
   18.0000
   

When I plot it just shows a straight line. How do I fix this? Thank you!

Answer 1

[John D'Errico]

p = polyfit(temp,carbonSteel,3);
y = polyval(p,carbonSteel);

What did you build? A model, that predicts carbonsteel, as a function of temp. So temp is the INDEPENDENT variable.

Now, you want to predict the dependent variable, as as a function of temp. So you need to do this:

y = polyval(p,temp);
plot(temp,carbonsteel,'bo',temp,y,'r-')

What you did, as you did it, makes no sense in context of the model you built..

[Christina Kersten]

Oh my bad that was a dumb mistake. Thanks!