test_polyfit.py  a x,y,5) 
x=
[ 0.   0.1  0.2  0.3  0.4  1.8  1.9]
y=
[  0.    6.   14.1   5.5   4.5   4.5   0. ]
polyfit p=polyfit(x,y,5)
[  21.86839417 -180.5352588   466.62341709 -451.84448956  127.5510778
   -0.49088898]
polyval y1=polyval(p,x)
[ -4.90888976e-01   8.19456248e+00   1.03966758e+01   8.29806717e+00
   3.70055224e+00   4.50391886e+00  -2.88758221e-03]
err=abs(y-y1) an array [  4.90888976e-01   2.19456248e+00   3.70332419e+00   2.79806717e+00
   7.99447762e-01   3.91886158e-03   2.88758221e-03]
sumerr=sum(err) a number 9.99309702111
generating plot test_polyfit_py.png