FSK constellation and eye diagrams
An eye diagram shows amplitude against time but FSK has nominally constant amplitude and carries zero amplitude information. Apart from that, an eye diagram is used for the raw data and not a carrier modulated by data.

A constellation diagram pin points regularities in amplitude and phase but FSK uses two frequencies therefore the phase relationship is lost on one of the two FSK frequencies and amplitude is constant anyway.

I would expect FSK to look like a dot or a circle in a constellation diagram (and possibly a partial arc of a circle) because FSK doesn't necessarily link carrier phase angle to the data changes (as is necessary in PSK).