I am new to creating graphs on excel - please help:

I have numerical data from cells A3 - A20 which is displayed graphically. If
I now insert a new row in A3 with a new data point, the graph does not
automatically update. Instead, the 'source data' now reads from cell A4 -
A21. How do I get the 'source data' to still read from A3 - A20, and drop the
value which now appaers in A21 (since I inserted a new row in A3)?
I have several of these graphs, and updating each one manually is not
practical.