Using graphviz vs smetana breaks tooltip on arrow. Uncomment pragma to confirm:
@startuml '!pragma layout smetana thing1 -- thing2 [[{This is shown on hover}]]: " " @enduml
http://www.plantuml.com/plantuml/svg/SoWkIImgAStDuL9NB2XAJCzDLCX9hCmlBL4epaqjISnBv2h9oCnBDrJGrLK0iumKegEhGp8oYnM0g3WZlpnF8JzF8IE_BBMeDZRMIa59GOdB8JKl1MWa0000
(--------)