Repeated values on average axis scale

Hi,

I'm using JasperServer Pro version 4.5.0 and have a problem creating Ad Hoc reports. Every time I create a graph of a field (e.g. civil status, single/married/partnership) against a measure (e.g. number of days in hospital), I get a strange display of values on the Y axis. With Sum, Min, Max, Count it works fine, but with Average I get repeated vales: 0 0 0 1 1 1 2 2 2 3 along the ticks on the axis. 

Any ideas why this happens and what can be done about it?

Many thanks in advance for your help and suggestions!