True it has something to do with choosing of coefficients. I managed to get a non-negligible but assymetric error band in the middle using sampling method for polynomial order 6 by changing initial coefficients . True lowering the polynomial order makes the error band wider but it makes it too wide.
However error band looks good when I use Chebyshev or Bernstein polynomials. However I am being unable to implement it for higher orders. I get the following error.
" In evaluate (/build/cmsbld/auto-builds/CMSSW_10_2_1-slc6_amd64_gcc700/build/CMSSW_10_2_1-build/BUILD/slc6_amd64_gcc700/lcg/root/6.12.07-gnimlf5/root-6.12.07/roofit/roofit/src/RooChebychev.cxx, line 125): Higher order Chebychev polynomials currently unimplemented.
Fatal: false violated at line 127 of `/build/cmsbld/auto-builds/CMSSW_10_2_1-slc6_amd64_gcc700/build/CMSSW_10_2_1-build/BUILD/slc6_amd64_gcc700/lcg/root/6.12.07-gnimlf5/root-6.12.07/roofit/roofit/src/RooChebychev.cxx’
aborting"
Is it only a root version issue or is there any other way to implement higher order chebyshev,bernstein in root 6.12 ?