As the graph rendering goes deep into PRTG internals, we don't support customization. This is mainly because of two reasons:
With the further development, we constantly improve the interface, making existing documentation obsolete. We don't have the resources to keep documentation about complex things as graph rendering up to date.
Modified graphs could break the webinterface layout.
There are undocumented files which allow some control over the rendering, however we currently don't provide documentation for this.
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