Time series irreversibility: a visibility graph approach L. Lacasa, A. Nuñez, É. Roldán, J.M.R. Parrondo and B. Luque Eur. Phys. J. B, 85 6 (2012) 217 Published online: 25 June 2012 DOI: 10.1140/epjb/e2012-20809-8