Cluster density distribution as a function of redshift in the zone of avoidance at |b_II|< 20°. Filled symbols are for the region with a Galactic hydrogen column density of n_H <  2.5 × 10²¹ cm⁻² and open symbols for n_H <  1.5 × 10²¹ cm⁻². The cluster sample in these region is not complete and therefore the data provide a lower limit. The density distribution has been normalised by the expected cluster density based on the average luminosity function as explained in Sect. 3.