Does Dynatrace provide dynatrace API that they can get into PostSQL database? Our customer like to draw a diagram which has to contain 5 different system profiles. I understand it can't be done. However, they are not willing to combine these 5 system profiles into a single one, which I believe the only way to resolve his requirement is asking if you do provide some API to use PostSQL data.
If you mean an API to the Performance Warehouse Database where the measures are stored, when no there is none. There are other ways to export data for external charting though, like the PureLytics stream.
What exactly is you use case though? You can build a dashboard which contains data from multiple system profiles, no problem. You just can't put multiple system profiles in one single chart.