Sonar only measures distance to the reflective object (by a time equation)...and echo strength (by how much of the sound pulse is returned to the xducer)...
It would show how far the jig is from the xducer (measured laterally-but displayed as depth on the display)...but not how far the jig is from the surface or the bottom (vertically in the water column)...
Now...if you can figure out a way to get the xducer to ride directly above the jig...this would show you how far down in the water column (depth) the jig is traveling...(because then the distance from the xducer to the jig would be equal to the water depth of the jig...minus how deep the xducer is in the water)...
Rickie