I have used dynamic ranges to create a chart that expands as new data is
added. I get economic data from an external provider which I update/download
daily or weekly. New data is added in new rows I refresh from the data
source and the chart expands automatically. The data is simply the Date in
column A and the value in column B.

However, I would like to chart the results of a formula in a column next to
the data from the source instead. For example, I would like column C to be
the percentage change from the prior period, which is what I want to chart.

Is there a way to extend the formula to the next row automatically as data
is added?