Anki is my flashcard program of choice. It outputs a lot of stats:
Early on in my Anki addiction I used several Anki decks at once. When I merged them, a lot of my early history with Anki (probably about 50k reps) disappeared. Because of this the early portions of these graphs look wrong. For example, there's not enough blue in the leftmost bars of the second and third graphs. Right now Anki allows you to merge decks without losing any of your history, but this wasn't always the case.
The above image was generated by Anki itself. I also produce my own Anki stats. It's relatively easy to do. The data file is an SQLite database. I opened it in the SQLite Database Browser and exported the main tables as text files. Then I linked those text files to an MS Access database, set up a few queries, pasted their results to Excel, and created some graphs.
I've organized my Anki cards with tags. The following graph shows the growth of each top-level tag through time. Vertical portions show wholesale import of cards.
This is the 50-day moving average of the percentage of reviews failed.
This is the 50-day moving average of the percentage of all of my cards that I studied each day.
And finally, the 50-day moving average of the number of reviews done each day.