The report for Project Velocity contains a graph and a few details of what is being counted.  The grouping for the report is set by the "actual finish date" field.  This grouping is used for the detail band and the graph.

In the data section of the chart, one of the series for the chart is set for a count of the actual finish date.  The other series are sums of the virtual fields (QOS, Task and Issue).    

The QOS, Task and Issues series are sums since we created the virtual fields to be a 0 of 1, depending on the Task Type of the record.  The line graph then displays a line for each task type that we have a sum for.  This should show you the Velocity for each task type, as well as a total velocity count.

Below the graph we have created a detail banded report that is grouped by actual finish date.  This shows the project manager and other members of the team if the project is gaining or losing speed.  If it looks like the project is losing steam and it should not be, then the PM should investigate.  Investigation can be done utilizing other reports or checking the Looking for blockades to development.  For a sample view see Figure 1 below.

This is the basis of the report.  This can be changed to track things such as Story Points, assuming you make that the task type custom field.

Figure 1