Could there be a plugin which shows the number of pages indexed by google?
You could do a google search for “site:domain.tld/folder” and use the number from “about xyz websites (0,12 seconds)” as displayed by google.
If possible, I would appreciate a trend graph for this as well.
I have seen that Yahoo indexed pages are shown in Piwik. For my website, Yahoo is far behind Google when it comes to indexing.
Yahoo has about 50 pages indexed but Google knows more than 600 - and it’s true: I can hardly find my website in Yahoo while I have several first page search results in Google.
I don’t exactly know where this comes from because I told Google and Yahoo to start indexing. People might be interested in this discrepancy.
On the other hand side I agree that this is more a general SEO problem than a web analytics topic.