Horizontal and Vertical gridlines are the lines that span wide and tall within the chart’s Plot Area. These gridlines originate from one of the axes — this makes the data easier to comprehend for the viewer. Gridlines may emerge from the horizontal and vertical axes — by default you can see only Horizontal gridlines on the chart. Following are a few points to be considered when you work with Gridlines.
Learn about Chart Gridlines in PowerPoint 2011 for Mac.

