  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 about 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
favors 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.
