node number evolution statistic?
Do you really need the publishing date? or just the created date?

If you need the former, well that date isn't tracked by core - http://drupal.org/node/1838918

You could try a contrib module to start tracking that for you... http://drupal.org/project/publication_date

If you're not worried about deleted nodes you can just do what you're after by running a query to count all the nodes that have a created date lower than month X. Or just get all nodes back, and then loop over them in php, successively incrementing counters for each month for your graph.

Otherwise, yeah you're going to have to do something with cron and get snapshots over time. I haven't heard of an existing offering for this in contrib.