Of everything on a journey map, the emotion curve is what executives remember. Nobody quotes the channel lane. Everybody remembers the dip.
Draw it per persona
A single curve describes an average customer who does not exist. Draw one for each persona on the map, in that persona's colour. A first-time buyer's checkout dip next to a returning customer's flat line says more about the problem than a paragraph could.
In Journima each attached persona gets its own draggable curve. The gap between them is usually where the work is.
Use a coarse scale
Minus two to plus two is enough. Nobody in a workshop can tell a six from a seven, and arguing about it wastes the room's energy. If you need finer resolution for research data, use a metric lane for the survey score and keep the curve for the story.
Anchor it in evidence
A curve drawn from memory is a hypothesis. Attach the interview quote that made you draw the dip. When someone challenges the shape (and someone will), the quote is one click away.
Do not smooth it
The temptation is to make the curve pretty. Resist. A sharp drop at "shipping cost shows late" is the point. A gentle wave is a decoration.
Let it move
The curve should change when the journey changes. When the shipping cost moves to the product page, drag the waypoint up and note the date. A curve with history is the cheapest before-and-after study you will ever run.