Update HoverData to include pixel positions.

Now includes array of Points corresponding with the elements being
hovered over.

Preparation for adding crosshairs.
I don't think it'd be any clearer:
```Typescript
points.push( this._isVertical ?
  {
    x: projectors["x"](d, i) + projectors["width"](d, i)/2,
    y: projectors["y"](d, i) + (projectors["positive"](d, i) ? 0 : projectors["height"](d, i))
  } :
  {
    x: projectors["x"](d, i) + (projectors["positive"](d, i) ? 0 : projectors["width"](d, i)),
    y: projectors["y"](d, i) + projectors["height"](d, i)/2
  }
);
```