You just need to move the value you want into msg.payload and feed it to the chart. So if you wanted to chart the longitude, for example, you would need a Change node that Moves msg.payload.iss_position.longitude To msg.payload.
As you are just starting with node-red I recommend watching this playlist: Node-RED Essentials. The videos are done by the developers of node-red. They're nice & short and to the point. You will understand a whole lot more in about 1 hour. A small investment for a lot of gain.
