Dynamic Normalization Chart ?

I have set up a dynamic normalization chart that tracks two stock prices. You pick a time of interest and it will zero out both stock prices to that time so you can then see how the price patterns converge/diverge from each other.

The chart dynamically updates when the stock price data is included within the worksheet but becomes static when the stock data is being externally linked. Is there anyway to keep the chart dynamic when the stock data is being externally linked? More specifically in the attached spreadsheet, if ETFC and GM tabs are saved as separate spreadsheets the chart is no longer dynamic.

Does anybody have any ideas how to keep this chart dynamic when the stock data is being saved in a separate spreadsheet?

Thanks,
Impala096