CrossTab by hand (graphing)

O this is a nice mission . I need opinion as much as code in this issue, so thinking hats on please.

I have a dataset, which I am using to fill a graph with. The graph is a time based setup in which I need to return results from a database based on certain (changable) paramaters. Heres an example:

EntryName EntryDate EntryValue
-----------------------------------------------------------
PC1 1/1/2003 14:00 5
PC1 1/1/2003 14:01 5
PC1 1/1/2003 14:02 4
PC1 1/1/2003 14:03 5
PC2 1/1/2003 14:00 6
PC2 1/1/2003 14:01 6
PC2 1/1/2003 14:02 6
PC2 1/1/2003 14:03 6


The graph will have 2 series, one for PC1 and one for PC2.
As you can see that kind of data willl graph quite easily, every minute a reading is taken, and saved to the database. The problem comes when a reading (or many) is missed.

A graph normally crosstabs the data and any entires that are in one series but not in another are blanked out (which is great), and you get gaps in the graph.

I want to be able to "crosstab" the data manually myself, in order to make sure there is a value (be it zero) for a constant range of dates. Say I pick 1/1/2003 all day, with an interval of 5 mins. I need a way to look at the records in each 5 min period of the whole and get the average value of that period, and stick it into the graph.
I may also want to get the data on an hourly scale (for a larger overview), so in each hour I need to be able to get the average value of all the records in that hour, and again, throw it at the graph.


Tis long winded I know, so thanks for any help, im hoping some1 has done something similar. So to the questions:

Should my graphing tool do this crosstab on my behalf ?
What graphing tools do you use and recommend that can do it ?
Is there any way to use the .net data functions to be able to do this easier ?
Could this be done by my database (its SQL or access or anything else we decide to move to, so must be generic functions) ?
Any other tips ?

Thanks in advance