Add hierarchy diagram

Author: Pat Altimore

articles/iot-edge/iot-edge-as-gateway.md

@@ -1,10 +1,10 @@
 ---
-title: Gateways for downstream devices - Azure IoT Edge 
+title: Use Azure IoT Edge as a gateway for downstream devices
 description: Use Azure IoT Edge to create a transparent, opaque, or proxy gateway device that sends data from multiple downstream devices to the cloud or processes locally.
 author: PatAltimore
 
 ms.author: patricka
-ms.date: 04/29/2025
+ms.date: 07/09/2025
 ms.topic: concept-article
 ms.service: azure-iot-edge
 services: iot-edge
@@ -39,7 +39,28 @@ For more information about how the IoT Edge hub manages communication between do
 
 Beginning with version 1.2 of IoT Edge, transparent gateways can handle connections from downstream IoT Edge devices.
 
-<!-- TODO add a downstream IoT Edge device to graphic -->
+<!--
+Change arrow styles to include a double hyphen to make it work in the markdown mermaid renderer. The single hyphen is a workaround for the HTML end comment tag.
+
+```mermaid
+graph TD
+    A[IoT hub] -> B[IoT Edge gateway]
+    B -> C[Downstream device 1]
+    B -> D[Downstream device 2]
+    B -> E[Downstream IoT Edge device]
+    E -> F[Child device 1]
+    E -> G[Child device 2]
+    style A fill:#f9f,stroke:#333,stroke-width:4px
+    style B fill:#bbf,stroke:#333,stroke-width:4px
+    style C fill:#bfb,stroke:#333,stroke-width:4px
+    style D fill:#bfb,stroke:#333,stroke-width:4px
+    style E fill:#bbf,stroke:#333,stroke-width:4px
+    style F fill:#bfb,stroke:#333,stroke-width:4px
+    style G fill:#bfb,stroke:#333,stroke-width:4px 
+```
+-->
+
+:::image type="content" source="media/iot-edge-as-gateway/edge-gateway-relationship.png" alt-text="Diagram showing the device parent and child relationship using IoT Edge as a gateway":::
 
 ### Parent and child relationships