Grease Pencil → Draw Mode
Select Draw tool
Open Brush Settings (Tool Settings or Sidebar → Tool tab)
Enable Size Pressure (pen icon next to Radius)
Click the little carrot (▸) next to Size Pressure
Boom — the pressure curve graph appears. It looks like its just a UI update where it doesn't plop the graph down in settings when you enable pen pressure. It did. Take me an embarassingly long time to find this in Blender 5.1
