Values of a,b,siga,sigb,chi2,q:
 Fit with x and y errors gives:
    -4.781926    1.987023    0.298884    0.021967   40.801914    0.055992

 Setting x errors to zero gives:
    -3.861928    1.953773    0.136495    0.009377  578.978271    0.000000
 ...to be compared with fit result:
    -3.861922    1.953773    0.132466    0.008947  578.978210    0.000000

 Setting y errors to zero gives:
    -4.876760    1.989112    0.179945    0.015475   59.940308    0.000414
 ...to be compared with fit result:
    -4.887378    1.990424    0.160903    0.015592   59.933216    0.000415