I attached an example to get the constellation diagram for a Duobinary signal.
To get a constellation diagram, we need both amplitude and phase information of the signal. The optical receiver can only get the amplitude information. Therefore, we need to use coherent receiver to get both the amplitude and phase information. The coherent receiver has no relationship to the modulation format, but in a future release there will be a universal coherent receiver. For now, in the attached example, I used a QPSK coherent receiver.