I assume you mean that it shows this: E.Injector|Casing Pressure
If so, this is the default way ProcessBook shows Element relative attributes. The only thing checked off is "Tag Name" in the trend. I'm using ProcessBook 2012, and even in this, latest version, the dialogs say "Tag Name" even though I've selected AF2 or Element relative as the data source for the trend. Hopefully the AF/ERD support will be better in future versions.
The E is present to let you know that you have Element relative data, and after that you get the attribute. In your case you probably have a sub-attribute named "Casing pressure" under the attribute "Injector". You should move the sub-attribute up one level to avoid having the long path. Then you would at least get "E.Casing Pressure" in the legend.
Using sub-attributes does make your attributes look more tidy in PI System Explorer, but I'd rather recommend you make use of the attribute category feature - and then check off the checkbox in the top right corner to "group by category". You could then assign a category named "Injector" to the "Casing pressure" attribute.
Thanks Asle, I really appreciate your advice! I'm new to AF.