The data usage overview has stopped working for me as well in mid-June.
I normally use around 500GB/month, but as you can see from the graph and table data, June was drastically lower with no change in our habits, and July shows nothing (today is July 8th, so well past the 24 hours needed to show data usage).
Something is broken on the Comcast side of things.
Here is a screenshot of what I'm referring to:
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