I was having a similar problem with a flowchart. I was bring a line down from an upper node and wanted it to terminate into an existing vertical line that came out of the bottom of a decision diamond.
It "appears" that I accomplished that. I drug a ball shape onto the graph made it fairly small and centered it on the vertical line. Then I connected my line from above to the ball. Lastly I shrunk the ball smaller than the line width.
It's so small now that I have to drag a selection rectangle over it to snare it if I want to adjust its placement. I suppose there are better ways but this is my first go at it.