Colour-coded relative errors in the exponent derived with PF (panel a) and PFL^* (panel b) due to misalignment of the analysed images. Each Ca II K image was randomly shifted in both x and y directions by up to 10 pixels. The boxes give the average error after 1000 realisations. The y axis gives the value of the maximum possible offset in any direction. The x axis indicates the analysed images ordered by date covering the period 2010–2016. Colour bars show the relative errors in the computed exponents. Over-plotted with white are the plage areas derived from the Ca II K images. The areas were scaled in the range [0,9] and are shown to indicate the activity level.