We report on 3.5 years of Chandra monitoring of the Galactic Centre magnetar SGR J1745 - 2900 since its outburst onset in April 2013 .
The magnetar spin-down has shown at least two episodes of period derivative increases so far , and it has slowed down regularly in the past year or so .
We observed a slightly increasing trend in the time evolution of the pulsed fraction , up to \sim 55 per cent in the most recent observations .
SGR J1745 - 2900 has not reached the quiescent level yet , and so far the overall outburst evolution can be interpreted in terms of a cooling hot region on the star surface .
We discuss possible scenarios , showing in particular how the presence of a shrinking hot spot in this source is hardly reconcilable with internal crustal cooling and favours the untwisting bundle model for this outburst .
Moreover , we also show how the emission from a single uniform hot spot is incompatible with the observed pulsed fraction evolution for any pair of viewing angles , suggesting an anisotropic emission pattern .