This tutorial may be particularly interesting to anyone who plots a series of data points over time with Excel. Using these instructions, one can mark “points of interest” along the X axis of an Excel chart in order to give context to the data. The author illustrates this by plotting his RSS subscribers over time, and the inserting a vertical line that shows the point at which is site was featured on a popular blog. The line can help to explain fluctuations in your data, and can be especially helpful when you are putting together a report that will be read by other people.