What is the best way to create a SSRS 2005 Line Chart Report for a 12 month period?

Question

I'm looking for advice on how to create a SQL Server 2005 query and line chart report for SSRS 2005.
I need to display the peak number of patients assigned to a medical practice each month for a 12 month period based on the end-user selecting a
single month and year.
I've previously created a report that displays all patients assigned to the practice for any single month but I’m looking for advice on how to
how to produce a resultset that shows the peak number of patients each month for a 12 month period. I thought about creating a query that returns the peak count for each month (based on my previously created report which displays all patients assigned to the
practice for any single month) and then use a UNION statement to join all 12 months but I'm sure that isn't the most efficient way to do this. The other challenge with this approach (twelve resultsets combined via a UNION) is that the end-user needs to be
able to select any month and year for the parameter and the report needs to display the 12 month period based on the month selected (the month selected would be the last month of the 12 month period).
For the report I’ve previously created that displays all patients assigned to the practice for any single month, the WHERE statement filters the
resultset on two fields:
Start Date - The date the patient was assigned to the practice. This field is never null or blank.
End Date - The date the patient left the practice. This field can be null or blank as active patients assigned to the practice do not have an End Date. When the patient
leaves the practice, the date the patient left is populated in this field.
Using these two fields I can return all patients assigned to the practice during Nov 2012 by looking for patients that meet the following criteria:
start date prior to 11/30/2012 (using the last day of the month selected ensures patients added mid-month would be included)
AND
end date is null or blank (indicates the patient is active) OR the end date is between 11/1/2012 -11/30/2012 (returns patients that leave during the month
selected)
Regarding the query I need to create for the report that displays the peak count each month for 12 months, I'm looking for advice on
how to count patients for each month the patient is assigned to the practice if the patient has been assigned for several months (which applies to most patients). Examples are:
John Doe has a start date of 6/01/2012 and an End Date of 10/07/2012
Sally Doe has a start date of 8/4/2012 and no End Date (the patient is still active)
Jimmy Doe has a  start of 7/3/2012 and an End Date of 9/2/2012
Given these examples how would I include John Doe in the peak monthly count each month for May - October, Sally Doe in the peak monthly count for
August - December and Jimmy Doe in the peak monthly count for July – Sept if the end-user running the report selected December 2012 as the parameter?
Given the example above and the fact I'm creating a line chart I think the best way to create this report would be a resultset that looks like
this:
Patient Name              
Months Assigned
John Doe
June 2012
John Doe                     
July012
John Doe                     
Aug 2012
John Doe                     
Sept 2012
John Doe
Oct 2012
Sally Doe                     
Aug 2012
Sally Doe                     
Sept 2012
Sally Doe
Oct 2012
Sally Doe                     
Nov 2012
Sally Doe
Dec 2012
Jimmy Doe                  
July 2012
Jimmy Doe
Aug 2012
Jimmy Doe
Sept 2012
From the resultset above I could create another resultset that would count\group on month and year to return the peak count for each month:
June 2012 - 1
July 2012 – 2
Aug 2012 - 3
Sept 2012 - 3
Oct 2012 - 2
Nov 2012 - 1
Dec 2012 - 1
The resultset that displays the peak count for each month would be used to create the line chart (month would be the X axis and the count would
be the y axis).
Does this sound like the best approach?
If so, any advice on how to create the resultset that lists each patient and each month they were assigned to the practice would be greatly appreciated.
I do not have permissions to create SPs or Functions within the database but I can create temp tables.
I know how to create the peak monthly count query (derived from the query that lists each patient and month assigned) as well as the line chart.
Any advice or help is greatly appreciated.

Answer 1

Thanks for the replies. I reviewed them shortly after they were submitted but I'm also working on other projects at the same time (hence the delayed reply).
Building a time table and doing a cross join to my original resultset gave me the desired resultset of the months assigned between dates. What I can't figure out now is how to filter months I don't want. 
Doing a cross  join between my original resultset that had two dates:
08/27/2010
10/24/2011
and a calendar table that has 24 rows (each month for a two year period)
my new resultset looks like this:
I need to filter the rows in yellow as the months assigned for stage 3 that started on 8/27/2010 should stop when the patient was assigned to stage 4 on 10/24/2011.
You'll notice that Jan - Sept 2011 isn't listed for Stage 4 assigned on 10/24/2011 as I included a filter in the WHERE clause that states
the Months Assigned value must be greater than or equal to the date assigned value.
Any advice would be appreciated.