Charts are a visual representation of data, and when seen on a PowerPoint slide from afar, you make out the value of a series by exploring where exactly it’s placed vis-à-vis the axis. Of course, if your chart has data labels, then this is less of an issue — however, typical charts have no data labels. For such a chart, you’ll find that Tick Marks placed on the axis will help us get a better idea.
Learn about the Major and Minor Tick Marks on Chart Axes in PowerPoint 2010 for Windows.

