Sometimes when there are two volume nodes on a track, they are connected by a solid diagonal line and the volume change is gradual.  Currently, however, there is a dotted horizontal line from one node that suddenly drops down vertically at the next node, making an abrupt volume change.
 
How do I control which behavior I get?

I always thought this had to do with clip editing, where the option to include automation was checked. I don't think I've ever been able to intentionally edit automation and end up with this result.
I know this doesn't really answer your question.

Move the cursor over the segment of the envelope that lies between the two nodes until the double-ended arrow appears, then right-click the envelope to choose the shape type.

I think the dotted line just means you haven't explicitly added automation in that area / between those nodes.   For example, if you add a node along the dotten line you should see one portion turn solid green.   So as scook said, by changing the segment type itself you're now designating what it should be.
 

Instead of adding something, just right-click on the dotted line and select one of the options, e.g. "Linear".  This will change it to a solid line.
 

stevec
I think the dotted line just means you haven't explicitly added automation in that area / between those nodes. 

Steve and Scook are on it. Keep in mind that if you play the track from the beginning, when you reach the dotted line section the volume will remain where it was at the last automation node across the dotted line and then when there is another node it will jump to that node.
 
When you start playback in the middle of the dotted line segment, the volume will not jump to the position of the last automation node. In fact, the volume could be anywhere on the continuum based on where you last had it.