Initially proposed by Edward Tufte, 'sparklines' present hundreds of data points in the space of a word or two. Tufte originally designed sparklines to be embedded in a sentence. Today they have moved off the printed page into websites, online applications, smart phone screens and interactive documents. Sparklines display hundreds of data points over time: stock prices or sports statistics for the prior year, for example. But how well do they perform with millions of data points acquired in microseconds? What if users capture these data every couple of minutes? How well do sparklines, primarily designed for static display of historical data, fare in the context of an interactive application? In this case study, the author describes interactive sparklines his team designed and developed to assist electronic engineers debugging their electronic circuits. The case study presents an iterative user-centered design process from the initial proposal of sparklines through to their refinement after several releases. The study concludes with reflections about future improvements to interactive sparklines.