Generating burndown charts using org mode

[Marco Craveiro <marco.craveiro@gmail.com>]

Hi Org-Moders,

I've been using Org-Mode quite a bit, but I'm afraid I'm not an advanced
user. I'm trying to generate burndown charts [1] which basically follow
coding progress over a bounded period of time (an iteration).

I started by generating a file for each iteration:

<snippet_of_org_file>
* Tasks in iteration
- [[file:list_1.org]]
- [[file:list_2.org]]

#+tblname: test_chart
| Day | Expected | Actual |
|-----+----------+--------|
|   0 |        4 |      4 |
|   1 |        3 |      4 |
|   2 |        2 |      1 |
|   3 |        1 |      0 |
|   4 |        0 |      0 |
</snippet_of_org_file>

list_1.org and list_2.org contain a total of 4 tasks for this time
period (tagged with :iteration1:). The table is updated manually at
present. "Day" is an index to the days in the time period, with work
always starting on day 1; "Expected" is the number of tasks expected to
be remaining on that day; "Actual" is the number of tasks actually
remaining. I then use org-babel/gnuplot to plot the chart as explained
on a previous post [2].

It seemed logical to try to generate the table automatically. I can use
org-mode's spreadsheet's facilities for generating all the days in the
table and I can even use date arithmetic to get dates for a day index.
As you can imagine, the bit I'm really stuck with is how to populate the
"Actual" and "Expected" columns. 

I've read a little bit about agenda views, and they seem to do a lot of
what I want, but unfortunately the lisp is not entirely obvious for a
newbie such as me. My questions are:

- is there an org-mode function that computes the total number of tasks
marked with a given tag in this file and any other files it includes?
- is there an org-mode function that computes the total number of tasks
marked with a given tag and marked as CLOSED on a given day in this file
and any other files it includes?

I was wondering if you could give me some pointers on how to go about
finding or even creating these functions as I failed to find anything
obvious. Even a "give up, its not trivial" would do, so I can at least
stop searching.

Many thanks for your time.

Marco

[1] http://en.wikipedia.org/wiki/Burn_down_chart
[2]
http://lists.gnu.org/archive/html/emacs-orgmode/2011-12/msg00813.html