Well I've finally got it working, but it seems to be a bit of a kludge.

Instead of putting all the data including the totals on 3 rows (my 3
series), I have the monthly data on the first 3 rows, and the total
data on the next 3 rows. Now I chart all 6 rows, and set the 3 total
row series to use the secondary axis. Finally change the colours of
these second 3 series to match those of the first 3 series and it works
as required.

Thanks for your suggestions Jon.

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