how to display current playing position in waveform~?

All you need is a 0. to 1. ramp that relates to playback (ie. the sync output of groove~ converted to a float)

Then plug that into either a [line $1] message or [prepend line] and plug into [waveform~].

Or just plug the float ramp into a multislider with 1 slider and a range of 0. to 1.