How to get a workflow diagram in Alfresco?

Question

I have created a workflow in Alfresco, but when I call the following service, after having started the workflow, to get a diagram for the workflow there is no diagram.

/alfresco/service/api/workflow-instances/{workflow_instance_id}/diagram

How do I get Alfresco to generate this diagram?

I am using Alfresco version 4.2.e.