[R] Counting session days

[stefan.petersson at inizio.se]

hi,

I have some session data in a dataframe, where each session is recorded with a start and a stop date. Like this:

session_start	session_stop
===================
2009-01-03	2009-01-04
2009-01-01	2009-01-05
2009-01-02	2009-01-09

A session is at least one day long. Now I want a dataframe with 'active sessions' per date. Like this:

date	active_sessions
=============
2009-01-01	1
2009-01-02	2
2009-01-03	3
2009-01-04	3
2009-01-05	2
2009-01-06	1
2009-01-07	1
2009-01-08	1
2009-01-09	1

How do I do that? I've searched the usual sources, but my newbie status and language barrier left me with nothing. So plz, anyone?