Currently this is the case:
A web content is created and scheduled for a month later. If it has a default DPT, the publish date in the panel will be the creation date of the web content, but for a month it won't be able to gather any data. If it is linked to a specific DPT, the publish date will be that of the link and again it might be that there's a period when web content is not published although the graph considers it.
This is due to the fact that blogs and documents don't have display date, so we haven't had the opportunity to use it.
Also a detailed research is needed to define metrics for assets and pages as distinct.