try treating the category range the same as your data range.

=SERIES('Monthly data '!$I$2,('Monthly data '!$I$1,'Monthly data '!$U$11,'Monthly data '!$AG$1,'Monthly data '!$AS$1,'Monthly data '!$BE$1,'Monthly data '!$BQ$1),('Monthly data '!$J$11,'Monthly data '!$V$11,'Monthly data '!$AH$11,'Monthly data '!$AT$11,'Monthly data '!$BF$11,'Monthly data '!$BR$11),1)

Of course there is a limit, 1024 characters, to the length of the Series formula. So eventually you will extend that are not be able to add more dates.

You maybe better restructuring your data into contiguous ranges for charting