There is insufficient data for 2019/20, the period most impacted by disruption due to the COVID-19 pandemic; therefore only data for the subsequent three years is used.
Nid oes digon o ddata ar gyfer 2019/20, y cyfnod yr effeithiwyd arno fwyaf gan y tarfu yn sgil pandemig COVID-19; felly, dim ond data ar gyfer y tair blynedd ddilynol sy'n cael eu defnyddio.