PivotTable and Date Query

Hi all, i am new here so please be gentle - I have a spreadsheet which has essentially 3 columns i need to use:
* Date Created
* Due Date
* Completed Date

What i need to do is to create a bar graph showing the number of items (month on month total), average days overdue (overlay line), average allowed days to close (overlay line) - for the latter two, i have a calculated field which generates the days overdue using:

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This serves its purpose and allows further analysis, to work out the days allowed to close i have used the same formula, using due date and date created instead.

Sample date:

Date Created | Due Date | Completed date
01/01/2014 | 21/02/2014 | 21/01/2014
01/01/2014 | 01/02/2014 | 10/02/2014
01/01/2014 | 21/03/2014 | 12/03/2014
01/01/2014 | 17/04/2014 | 01/04/2014


The Graph theoretically needs to have:

* X Axis: Months (Jan - Dec)
* Primary Y Axis: Total items (for bar)
* Secondary Y Axis: Average Days (for 2 line graphs)

The total items should be a running month on month total ie. the total for Jan on the graph the bar should be 3 (i.e. total left form Feb - Dec), the total for Feb should be 2, the total for March should be 1 and the total for April should be 0. This is based on the Completed date and be a running total year to end.

I need the ability to set a starting number, which will be the number of items December 2013 total, i think i can do this using the "Show Data as Difference From" option on the PivotTable, however i can't seem to see a way to set a starting total?

I am using Excel 2010 on Windows 7.

I am unable to upload a sample at the moment, however if needs be i can tomorrow.