This button allows you to insert a Volume Analyzer (VA) line into your box chart, which will appear over the yellow boxes as a dashed blue line. The vertical location of the VA Line is irrelevant (all you're interested in is the overall performance, as summarized by the UV/DV label)
UV/DV = 0.84
In the picture shown here, the VA Line is shown with the UV/DV label at its right-most end reporting a UV/DV of 0.84.
After a VA Line has been inserted, you will observe a thumbwheel is attached to the VA Line button (this is shown in the figure above). "Spinning" the thumbwheel with the left mouse button lets you shift the VA Line up or down, allowing you to move it above or below existing graphics for clarity.