Adding an onLineEvaluated_ to dygraphs kind of seems wrong to me because dygraphs is the one actually calling the evaluate() method. What would you think about one of these two solutions:

1.) calling an alternate callback:
this.layout_.evaluate();
this.dataHandler_.onLayoutEvaluated(...)
-> For this solution I would have to adapt my callback to be more general.

2.) adding a callback function to the layout.evaluate method.
this.layout_.evaluate(this.dataHandler_.onLineEvaluated)
-> the layout could then call the given callback in a more abstract way.