The Selected Data toolpart for RtTrend dictates what traces appear in the trend. It is an ordered list of PI Points, relational dataset columns, and web service parameters, and can also include one connection from another web part. This connection can consume any number of PI Points, passed as a semicolon-delimited list of the format \\piserver\tagname[;\\piserver\anothertagname]*
Often overlooked in the Selected Data toolpart for RtTrend is the "Replace Ad-hoc Traces" checkbox. This property controls the response to the web part connection for the Selected Data property. Checked by default, this causes every new batch of PI Points received via the connection to replace the last batch. In other words, by default PI Points sent via the connection to the Selected Data for RtTrend are substitutive. However, if you uncheck this property, PI Points sent via the connection will be additive -- the existing traces will remain while new traces are added for the new PI Points received via the connection.
For example, when the AliasTagList parameter from RtTreeView is connected to the Selected Data for RtTrend, and ad-hoc traces are not being replaced, each time you click on a Module in RtTreeView, the PI Points to which all of its Aliases refer will be added to the traces in the trend. No single PI Point will appear in the trend more than once, so if a subsequent connection event provides a PI Point that is already being traced, a second trace will not be added to the trend for that PI Point.
The RtXYPlot web part provides similar functionality, allowing additional "Y" tags to be passed in via connection either additively or substitutively. Hopefully calling attention to these features will help you get more value now out of your existing RtWebParts applications.