You can use these instructions to mark important points along the axis of an Excel spreadsheet chart.
“How To Add a Vertical Line to an Excel XY Chart” by The Closet Entrepeneur provides instructions for marking points of interest on an Excel chart with vertical lines.
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.