@Susan: Unfortunately, it is not possible to do this with the native functionalities of PI ProcessBook. I would see more about this from R software or Matlab.
Yes, it is possible to write-up your own ActiveX trend object but you will need a lot of hours to succeed. You will also need to use VB6/VC++ ATL. I have heard some customers had a lot of success with Silverlight embedded controls within PI ProcessBook.
I hope that this helps you!
Susan, Can you expand on why you want to look at your data this way? The digester number is a label; why are the different digesters in a time-plot? Does the line-up of digesters across time indicate the pulp flow? And, will it vary from day to day?
I'd expect that you would want to look at each digester as a separate batch process; please see PI Batch visualization tools - and also BatchView add-in for ProcessBook. Or, Event Frames.
I've written a few ActiveX add-ins for ProcessBook and it's fairly straight forward. There are resources in the forums which'll help you, and there are also hands-on learning labs from vCampus Live 2010 or 2011 (I think) which help you get started writing a ProcessBook add-in. That shows how to embed Silverlight controls.
I've mainly used .NET Charts (formely known as Dundas Charts) and these are easy to work with. I'm sure you'd be able to complete your challenge in 3-5 days.
Here's an example of one of the components I've created, which shows the load duration curves for a couple of measurements through-out the year: