Hello Julie,
There isn't a way to export the Velocity chart data into Excel. I also tested this with some of our super customizable Charts and exporting is not an option there.
Super Customizable Iteration Chart | CA Agile Central Help
If this something you would like to see I would suggest posting an idea to Ideas.rallydev.com.
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We encourage enhancement requests that other users would like to see and can vote on.
Please let us know if you have any questions.
Thank you,
Christian