Rename 'topAnnotationHolder'

- Rename 'topAnnotationHolder' as 'topAnnotationsHolder' in
  views/graph.js and update test script.
Move 'getNearestPoint' function

- Move 'getNearestPoint' function to lib/data_representation.js.
Refactor code

- Refactor code to avoid use of 'labelTextView' in views/graph.js and
  tools/label_state.js.