Each chart presents the years for which each assessment was in place, except for the year 2019/20 for which there was insufficient data due to the disruption caused by COVID-19.
Mae pob siart yn dangos y blynyddoedd yr oedd pob asesiad ar waith, ac eithrio blwyddyn 2019/20 am nad oedd digon o ddata ar ei chyfer oherwydd y tarfu a achoswyd gan COVID-19.