Suggestion: If you refer to an older chart, could you include in which update it was? Or perhaps just attach the chart again with the same number.
I find myself endlessly searching my mailbox.

+1
Maybe it's good to put the chart numbers in the email subject, like this:
Feb 29 update
(chart 61-62, 42-43)Everytime a email is sent, put the email subject in this thread. It helps to attract the new customers, they will guess the rich content as the subject suggested.