The 6 Percent: a weekly growth tool based on Paul Graham's essay

The function of calculating growth is not something you would need to call from this tool, as it should be easy to implement by yourself locally. I think the visualization part is the one you would want. I used to develop analytics for a startup and we would measure the whole range of freemium model of the service, but I guess the CEO would also have liked a simple single graph seeing overall growth (at least in terms of paying customers) at the end of a week. If you need more, there's tons of nice visualization frameworks nowadays helping you do all kinds of crazy stuff with your data.