Centre velocities and peak intensities of maser features in each association over time. The marker size is proportional to the square of the feature peak intensity ($\propto I_{\mathrm{peak}}^{0.5}$). A scale for the marker size is shown at the bottom left of each panel. The markers are colour-coded according to velocity. The grey horizontal lines indicate the dates of our observations. The date of the onset and peak of the accretion burst traced by 6.7 GHz methanol masers is shown in red and black, respectively (MacLeod et al. 2018).