• It is Spider Project, but P6 should equally be able to deal with portfolios with many resource pools and their reporting. 
• While certain project can only access to project pool other projects perhaps because of geographical location [municipality/town/country/region] might have access to other pools. This can complicate the creation of histograms where some of these parameters are only [but easily] accesible via the tabular reports. 
• I  expect P6 to deal with these reports if using tabular reports making it very easy for Pierre to get the basic portfolio histograms he wants and more.
• I believe this is what David Kelly does even for the simple case presented by Pierre where only the Global Pool is shared, so easy.  A one [Global] pool to many projects resource distribution instead of many pools to many projects resource distribution.  It is not uncommon for close projects to share some resource pools, many pools to many projects.  It is not uncommon for some pools not be available to distant projects.

Within large portfolios you might many projects plus different shared pools that migh supply same skill to selected projects:

1. Project Pool
2. Global Pool
3. Subsets of Global Pools

In such case tabular reports are essential,  you need to know from which specific pool and to which specific job the resources are to be moved. When you have many projects running at the same time, each demanding resources from its own set of resourc e pool it is not practical to use histograms.

Pierre,

As you have already found, the filtering in the stacked histogram is by attributes of the Resource Dictionary only, NOT the resource assignment. That it will only stack "At completion" rather than "Remaining" Units is perhaps an issue you didn't quite get to......

The way that all of my cleints get around this is to use the Report Wizard to build a pivot table directly into Excel, and chart wizard your solution....

No doubt knowing the resource allocation per job/project for every resource when your portfolio share resources among different jobs is essential to good resource management.  With a report template it is so easy, make use of your software reports if there is no such view available.

Following your suggestion to look for project code parameter but using different software it came out easy if using Tabular Reports and Excel as not all parameters are available for Histogram/Graphical Reports.  I suggest you try similar procedure using your software.

1. I created a tabular report template where project code parameter is accessible.  It took a few extra clicks to create the template but it was very easy.  The template allows me to re-create the report at a single click of the mouse.  Click Add and select any additional parameter from dialog box as required, in this case Project code.
2. Using the template I generated a tabular report that gave me the required data for the histogram and exported to Excel.
3. Within Excel the Staked Histogram was created with a few mouse clicks, easy.
4. In my case I would make it even easier by using a Resource Gantt, a view not available in P6, in this case the report will come out without the need to look for project code parameter.  Maybe you can generate the report from the resource usage table. The source data for the required histogram is just a resource usage table organized by projects, essential and very easy.

Good Luck.

what version are you using ?

I think that if you assign a different ROLE for each resource for each project that you will able to do this.

the resource can be the same but the role can be different