744 - Projectors w/data arg takes 1st datum - area/line 

Close #744.
also, why not just override generateAttrToProjector with this logic? 

so like
```
public _generateAttrToProjector () {
  var attrToProjector = super._generateAttrToProjector();
  return this.projectFromFirstDatum(attrToProjector)
```

or something similar. note you could then make the method private and the change is better encapsulated