The upgrade from net-analyzer/rrdtool-1.5.5-r1 to v1.6.0 triggered a preserved library event for app-admin/collectd and sys-cluster/ganglia. Please set sub slot to SONAME version which would allow consuming applications like collectd or ganglia to set an slot operator on net-analyzer/rrdtool so that these packages can be rebuild automatically when necessary.
Fixed in -r1.