Add a line to the plot representing a quantile fit
df
Degrees of freedom of the quantile regression
proportion
Quantile to fit
col
Color for plotting points
col.rq
Color for plotting quantile line
References
Calza et al., 'Normalization of oligonucleotide arrays based on the least variant set of genes', (2008, BMCBioinformatics); Pawitan, Y. 'In All Likelihood: Statistical Modeling and Inference Using Likelihood', (2001, Oxford University Press); Huber, P. J., 'Robust estimation of a location parameter', (1964, Annuas of Mathematical Statistics).