text: Title: Error on hover event of parallel coordinates graph component
  Body: I am trying to associate a callback to the hover event in a parallel coordinates plot, but it does not seem to work properly. More specifically, it raises the following JS error in my Dev Console:
  ![length_not_found](https://user-images.githubusercontent.com/1186798/35578518-9e1a1612-05e4-11e8-834b-7bfc3eac7f63.png)
  
  Any clue?