Correlation of medium-term flux density variations in ALMA and ACT light curves of PKS 2131–021. Top: ALMA 91.5 GHz and ACT 95 GHz (PA5) light curves that have been filtered by binning and subtracting a long timescale trend. The binning is in five day chunks, and only time chunks containing data from both ALMA and ACT are retained. The long timescale trend was removed by fitting a third-degree B-spline with 120-day knot spacing to each binned light curve and subtracting; this effectively acts as a high-pass filter of variations on timescales ≳120 days, removing the large sinusoidal pattern. Clear correlations between the light curves are seen on these timescales. Bottom: Pearson correlation between the filtered ALMA and ACT light curves, for different spline knot spacings. Errors were estimated by calculating the correlations between the ALMA PKS 2131–021 light curve and each of the other 204 ACT light curves in our bright AGN library, which should be uncorrelated, and taking the standard deviation. The thick coloured point at 120 days corresponds to the data shown in the upper panel.