If you read blogs in Bloglines you probably noticed that on the line with the blog name you can also see how many subscribers a blog has. It's not the best, but it is one of the easiest way to predict how popular a blog is. I checked the Bloglines API to see if there is a way to get this number periodically, but apparently there is not. Too bad...
But there is a way to get a number for your blog if you have access to web server log files of you blog! When Bloglines spider checks your site (every 2 hours or so) it leaves a trace in your log files. And in that trace they also leave a count of subscribers to your blog in the user agent field.
So if you use LogParser and run a query like this:
SELECT MIN(date), cs(User-Agent)
WHERE cs(User-Agent) LIKE '%bloglines%' AND
cs-uri-stem LIKE '%rss.asmx'
GROUP BY cs(User-Agent)
you will get a list of dates when you subscriptions count changed. (You would probably need to trim some parameters like log filenames and filename of you rss/atom feed, but it should be too hard).
These are sample results:
... and so on...
The full command line to get result of a query to a CSV would look something like this:
H:\>logparser "SELECT MIN(date), cs(User-Agent) FROM F:\logs\w3svcXX\ex*.log
WHERE cs(User-Agent) LIKE '%bloglines%' AND cs-uri-stem LIKE '%rss%' GROUP BY
Now you can copy this files to Excel, create a formula to extract the number from user agent field and you are more than ready to draw a 3D graph! (if numbers are not enough for your blog-ego)