This report is called Issues by Status fixed version, and on this report we show statuses of each version that are unreleased. Unreleased versions are the project versions that have been created but not yet released. A single bar in this report works the same as the Issues by status category where each issue is categorized by its status (Todo, InProgress, Done).
For instance we have two bars on the above chart representing two unreleased versions. The names of these unreleased versions are shown on the Y-axis the bar (v1,v2)
When you hover over a bar chart a tooltip appears that will show the number of issues in the status and the name of the unreleased version.