You are traversing through a collection and adding data in dataCol instead in item. like
For Each item In bigCol
dataCol.Add(New PlotData("name" & i.ToString))
i += 1
Next
In this code you are traversing bigCol then the item is not being used anywhere, It should be like this.
For Each item In bigCol
item.Add(New PlotData("name" & i.ToString))
i += 1
Next
Then again while you are displaying you are doing the same mistake of displaying.