From what I can gather, a "sparkline", is a small, word-sized graph, which is useful for visually charting data within the text of your analysis, rather than as a separate figure.
bumppo.net uses a sparkline "depicting a recent 12-month window of my Netflix rental history, where red lines indicate months in which my average rental cost was higher than $2.00, and black less than $2.00. (Netflix's monthly rate shifted twice over the last year, explaining the small discrepancy you may notice in the last line.)".
Don't ask me how to make a sparkline. I have no idea.