Fix hover behaviour on Sankey diagrams

* Tooltips are now positioned outside the rectangular node, and will not
  move with the mouse. Tracking the mouse seemed to interfere with the
  tooltip and frequently caused its disappearance.

* Set the "viewport" and "adjust.method" options to ensure that tooltips
  remain inside the chart; prevents tooltips from the left-most nodes
  from being rendered off-screen when using the "Zoom" button.

Fixes #1840.