Chebychev Polynomial fitting results show multi lines

[weitong]

I am using "nonliear curve fit.vi" with defined Chebychev Polynomial  function to fit XY data.  Strangely, if I enlarge the upturn part, I clearly see the fitting results show multi-lines (2-4) . Please the the attached snapshot.

weitong

LV2013

[P@Anand]

Can you please attach the actual VI?

[weitong]

Thanks.

 I found the reason: the data X values are not mononicaly, i. e.  there are local oscillations at some region.

After sorting the raw data, the fitting results show single line. 

Is the sorting of X values necessary before Chebychev Polynomial fitting?