Renamed to also note that filtering would be also nice

Something like this is definitely needed. My use case is that I track the time spent for each customer as a single activity and at the end of the month, I need to generate a report of what I did and how long it took. This report is sent together with my invoice.

So I need a report that sums up all facts of a single activity over a given period of time. In that case I don't need the Activity/Category columns and I want a single total (and not one per activity).

Yes, also I do not track non work tasks, reading online, eating, playing games, etc. because I don't want to deal with removing them from my invoices.

It would relly be helpful to set include / exclude at the Categories and the Activities level. A checkbox for each - "Exclude / Hide from report" - would give great granularity for reporting.

I'm not sure if it is better usability to set the "Exclude" bit at the category / activity in preferences, or if it would be better to have a checklist / filter section on a reporting dialog.

I think that maybe having it set at a report dialog (with settings saved from last session) may be better, as you could quickly export different types of reports, setting whatever filters and bits you wanted.

Wow, a very useful piece of software is becoming even more useful! I'm going to have to start learning python to contribute more than just feedback and bug reports..

I don't think we want to include a list with hundreds of checkboxes. Keep in mind the user is free to invent activities as he goes (just type a new name) so over time it can accumulate to huge numbers.

I propose having a separate app. Basically a wizard that:

(1) first asks you for time span to report,

(2) then on the second page asks for the categories you want (including only the categories you used during that period)

(3) and only then presents you with a list of matching activites so you can (un)check some of them.

Possible further steps include template selection etc. We could implement templates as plugins so you could pick from say "Professional", "Modern" as well as "CSV", "XML" or even "Initrode/Initech Custom Import Format, revision 6.B/1995".

Charles - you could easily track all those non-work activities in separate category, which is not the case for Raphael.
The work i did on the "save report" dialog does filtering by category and date range.

Good points by Patryk in #4, In my use case I don't have very many categories, I use a few main categories and many activities, and then activities vary in description. 

Toms, I just built a fresh one from GIT (nice stats btw) and have two comments on the filters:

1) They won't work for me in current state, because they are radio buttons instead of checkboxes. In my reporting, I have 3 categories that need to be on an invoice (1 employer, 3 websites that split time and work between, then multiple activities within each website category).  This allows time to be totaled for each site separately, as requested by my employer, a nice combination of total time and time spent on each. I could see how in #4 Patryk's use case checkboxes could be cumbersome to use, but could be overcome by a simple "check all / clear all" button to effectively have similar functionality to radio buttons but in 2 clicks instead of one.  I could generate 3 reports, one for each site, but I'd have to combine them anyway.

2) The categories filter does not wrap and prevents the dialog from resizing. Want me to file a separate bug for this?

RE: #2 Raphael's case (and all of ours), it sounds like the features and filters need to be the same for each column, treating categories, activities, and descriptions nearly the same (totals, filtering, excluding) simply because everyone uses each so differently. Complex! 

I love the date range!

filtering per date range, activity name, category, description and tags is now in git master. the save report dialog does not have any extra knobs any more as filtering is performed before that

report customization, as in which columns, what order and format and calculation and interpretation of totals, should be performed via spreadsheets (using TSV output), XSLT templates (using XML output) or directly consuming the d-bus API.

regards #1 of comment 6: the search input box does boolean searches, space is AND and comma is OR, pretty much like in natural language.
example:
"outer space invaders, ponies" will return results that are mentioning all three "outer", "space" and "invaders" in any of the fields, or just "ponies"

as we support two word searches for sake of two word tags, this actually is a bit more complicated, as it will go for (("outer" and "space") or "outer space") and so on, but that's not really relevant as it "just works"(tm).