Tracing lines on a graphic

I am creating some additional work for students in Storyline 3 that traces signal paths on an electronic schematic.  In most cases the trace being a little to the left or right of the line on the schematic is not a problem.  However, when space is limited, I need the traced line to be directly on the schematic's line.  I cannot seem to get the lines I have created for the trace to fall on the line in the schematic.  I have seen  finished products where this was done so I know it is possible.  I just don't know how.  Any assistance will be appreciated.