@Bastien_Siebman Custom Fields are actually global components that can be applied to projects and portfolios all the same. I quite often will use the same global Custom Fields (saved to Org) at both the project and portfolio level.
I believe what @Vanessa_Künstler is pointing out is there isn’t a way to see the average/sum of numerical values in custom fields from the portfolio view, like you can in projects across the whole or within sections.
I’m hopeful that in the future the Dashboard tab will be available for Portfolios and this would be a great way to see the breakouts you are looking for @Vanessa_Künstler. In the meantime, I would recommend looking into exporting your Portfolio to a Google Sheet.. This creates a dynamic connection and enables you to create a variety of visuals like pivot tables, charts, dashboard widgets, etc. While I, too, would love to see some level of charting like this in Asana it is a fantastic integration that syncs data every hour!