Solution
For creating a XY-Channel Pair you need to set your time channel as
XRelation for your Y-Channel. This can be done as followed:
dim oChnX, oChnY
set oChnX = Data.Root.ChannelGroups(1).Channels("Time")
set oChnY = Data.Root.ChannelGroups(1).Channels("Velocity")
set oChnY.XRelation = oChnX