The National Center for Education Statistics states that on a bar graph where the bars are placed vertically, the y-axis runs vertically from the bottom to the top of the graph. On bar graphs where the bars run horizontally, the y-axis is placed horizontally from left to right.
The y-axis is generally used for numerical measurements when comparing two or more items. The measurements on the axis usually begin at one, rising in evenly spaced sections along the axis to show the different sizes of the statistic that is being displayed in the graph. The name of each displayed item is shown on the connecting x-axis, which is placed at a 90-degree angle to the y-axis.