I assume you're talking about the "Device Chart" portlet type.

There are only two factors that control whether a graph point will be available to choose when a device class is chosen. The first is that the monitoring template containing the graph point is bound to the device class. This doesn't mean that the monitoring template was created in that device class. It specifically means that the monitoring template must be bound to the device class, not just the devices in the device class. It sounds like you're saying this is true in your case.

The second factor is that only "Data Point" graph points are supported. So if you've a "Threshold", or "Custom Graph Point" type of graph point, you won't be able to see it. You did mention a custom graph point, but those aren't typical, and I'm thinking you just mean custom in that you created it.