As far as I know, the data has to sit somewhere. Changing the value in one cell deletes the previous value in that cell unless the old value has been written somewhere else. The location of the historical data doesn't need to be visible on a sheet - but it does need to exist somewhere.

I would have thought you need two data points for each new value: The applicable date and the value on that date.

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Andrew Lockton
Chrysalis Design, Melbourne Australia