What do the curved lines in the graphs symbolize?

All over anki, both in add-ons, and in “vanilla” Anki, I see graphs with lines on the top of them and I can’t figure out what they represent/symbolize?

Thanks!!

In different ways. Somewhere everything is obvious and somewhere you need to think a little.
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The total % of the total number of cards.
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“Card stability” is the same as “Added”

The Graphs

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