I'm not really clear on what `chi_line` does.
I presume it's one method to take errors on `x` into account when fitting a 1-dim model to `(x, y)` data points.

Some reference I'm aware of:
http://adsabs.harvard.edu/abs/2010arXiv1008.4686H
http://root.cern.ch/root/html/TGraph.html#TGraph:Chisquare

Please add a reference and a test case with a few points for this fit statistic.

Also I would suggest to rename it ... to me "line" sounds like straight line, whereas I think this can handle non-linear functions `f(x)`?