Worksheet_Change is keyword to our search engine http://www.excelforum.com/search.php :-)
The code could be as simple as that:
There are 2 sheets Archive one and the one with data. lets say value is in C3 of data and in archive in columns A and B is excel table (Ctrl+T) with dates and values.
As these are in Table, the graph is updated every time new value appears in table. and it appears when above code is executed.
This code is in your sheet data code (right click on tab and choose "display code")
Try this
PS. if changes are more frequent (more often than daily) I'd personally change graph type to scatterplot.
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