read_data file=dat.dat, group = d, label = 'c g err'
a.c = log10(d.c)
a.g = log10(d.g)
set a.err = (log10(d.g + d.err) - log10(d.g - d.err))
set b.err = a.err * 100.

# g = m*c + b
guess ( m = 1 , b = 0)
fit.g = m * a.c + b
fit.resid = fit.g - a.g
minimize(fit.resid, uncertainty = a.err)
show @variables

minimize(fit.resid, uncertainty = b.err)
show @variables
