Left: difference of fractional plage area between those derived from centred and off-centred Ca II K observations as a function of time. The differences shown are for the Co-CoW (orange), MD1-MDW (green), MD1-MDV (blue), MD1-MDR (red), ML-MLW (ciel). Dots show daily values, while the solid lines show annual median values. The dotted horizontal line denotes a difference of 0. Right: distributions of the differences.