Top: Disc-integrated Ca II K intensities (photometric sum series, see Sect. 2.2). For San Fernando data the solid lines show the original series, while dashed ones are after linear scaling to match the photometric sum series from Rome/PSPT. Shown are 27-day running means, whereby the means are computed for all series within the same time intervals, even though these intervals might not include the same number of data points in all series due to their different availability. Bottom: Relative differences between the scaled photometric sum series as denoted in the legend. The dashed horizontal black line denotes the zero relative difference.