The following describes how you can mimic lines that do not end at a symbol. This includes lines ending at other lines and completely independent lines.
Every Diagram is a Graph
In yEd, every line has to have a start node and an end node. This way, every diagram is a graph which is required in particular for the layout tools. Depending on the specific use case, there are several work-arounds available. Each of these comes with some drawbacks.
A seemingly independent line can be created by using very small or invisible nodes (no fill colors and no line color) as the start and end nodes. If such a node is placed on another line, it appears that two lines connect to each other.
Layout algorithms have no special handling for invisible nodes and may produce unexpected results for such an arrangement. The position of an invisible node which models a connecting line has to be adjusted manually when the position of one of the related nodes changes.
Shared Line Paths
A line connecting to another line can be modeled also by shared line paths. To achieve this, create a regular line to the point where it meets the existing line and then follow the path of this line to one of its end nodes.
For convenient editing, you can use the mouse selection box to select all overlapping bends at a connection point and move a complete bundle of edges at once. Also, if you move a node, all of its edges with shared paths stay together.
Layout algorithms may or may not keep the edges combined.
Horizontal or vertical lines can be modeled by thin shape nodes.