Evolution of centers of classes computed with SPoCA2 on a frame-by-frame basis on EIT 171 Å (left) and 195 Å (right) images. The dataset consists of one image per bandpass every 14 days during the period 1 January 1998 until 31 December 2008. The blue solid lines represent the median value over the whole period, while the red dotted lines are the values found using the SOLMOD ratios.