Ok now it works on mobile too for all graph types. THANK YOU.
My bad. When Darren suggested using Maths columns to spilt and reform the date column, I applied that to the master data set only. Of course a relation has 2 fields, the field to match and the match options. I had only adjusted the latter.
With your last response, I realise the relation also starts with a field generated with use of a ‘format date’ column. I replaced that with the suggested maths column, e.g. month(now) and year(now). It worked first time.
The reason the pie chart was working is because it didn’t use the relation. It was looking at share of total rolled-up data and did not require the chart to be split by month-year (it’s a pie-chart!).
Again, thank you for your patience… appreciated.