If you chart contains more than one dimension then there are various stacking options available. There are four stacking options available in the visualisation widget, "Group", "Stack", "Percent" and "Drill down". 

In these examples with have a chart with two dimensions (Device and Medium) and one metric (sessions).

​Group

Here the metric is split by the first dimension and are then grouped by the second dimension. Each value has its own bar.

Stack

Again, here the metric is split by the first dimension and are then grouped by the second dimension. However, the segments for the second dimension are stacked on top of one another.

Percent

This stacking option visualises how much of the dimension is made up of the other in a percentage number format. 

Drilldown

This stacking option embeds the other dimension inside of the other. Firstly you will get the metric broken down by the first dimension and you can click into the dimension value to see how it is broken down by the other dimension.

Firstly, the chart will appear as a one dimension chart. However, by clicking on the dimension value. It will expand its segmentation. The dimension value is underlined, representing that it can be selected.

By selecting it, the second dimension becomes available. On the top right of the chart there is an indicator of which dimension value has been opened at a higher granularity. By clicking on it, it will bring you back one level of granularity.