Proportion of the solar disc covered by faculae, n_fac in the MDI (red) and HMI (black) data sets over the period of overlap. The solid red line gives the final result which incorporates the cross-calibration of the magnetogram signal and n_fac in the MDI data set (see text). The dotted red line shows the level if we omit the adjustment to n_fac and the dashed red line that from the original MDI data.