Currently the documentation reads: "When using the Infographics widget, you can use the related table as an external data source. When you do so, the related table is treated as a stand-alone table. There is no interaction between the table, related features and charts."
This is unfortunate. The Info Summary widget can filter by extent but doesn't have the charting capabilities that the Infographics widget does.
Please consider having the 'Filter By Extent' check box apply to related table records in the same way it already does for Feature Layers.