Once you close the Trend View, you lose all trend data, and the element set itself. If you save the project to Disk, the workspace will remember the Trend View elements and layout WITH THE PROJECT without having to actually save it to disk.
You can explicitly save the element set of the Trend View to disk. Under the Debug menu, select Trend View->Save As or Save. Then at a later date, you can bring back that specific Trend View by doing Debug->Trend View->Open.
Data View behaves similarly.