Spatial positions of IRIS burst profiles through time in two datasets. Left-hand panel: map of the Si IV 1394 Å line core intensity (logarithmically scaled) within AR 11850 sampled during the raster starting at 05:59:43 UT on 26th September 2013. Right-hand panel: same as the left-hand panel but for AR 11916 sampled during the raster starting at 03:09:49 UT on 7th December 2013. The contours and crosses over-laid on both panels denote the IRIS burst profiles and their centres-of-mass, respectively, for each raster containing more than 100 automatically detected IRIS burst profiles. The colours indicate different rasters and are consistent with those used in the right-hand panel of Fig. 4.