twotheta counts_per_second sigma(counts_per_second)
(maybe, this is what you mean).
This has a very good advantage: The measured data can consist of points
with different accuracy. In principle, the measurement can consist
of some non-equal spaced measuring points with different measuring time
and counts for every point! This is the basic concept of experimental
design. I have measured and fitted such data, first in 1983.
Joerg Bergmann
bergmann@rcs.urz.tu-dresden.de