This is an Octopus curve tracer circuit. It has been around since the 1930's. The blue output is the Y axis which represents current and the green is the X axis which represents voltage. If you put an oscilloscope in XY mode and connect blue to Y and green to X you should see a a diode curve with the diode that is in the circuit under test. I have a Tektronix 475 so I have built myself a curve tracer and verified that it works.
