I haven't used ZEDGraph for years so I don't remember the exact details, but the trick to a scrolling graph is to add a new point, then remove the oldest from the point collection when the count exceeds a certain value. This is how to do it with Microsoft Chart:
Dim p as double
p=EthernetIPforCLXCom1.ReadSynchronous("MyTag",1)(0)
Chart1.Series(0).Points.Add(p)
if Chart1.Series(0).Points.count>100 then
Chart1.Series(0).Points.RemoveAt(0)
end if