Plots: ensure hover tooltips do not clip near the plot boundaries

[jacobvjk]

CONTEXT

When approaching the chart boundaries in the MultiLineChart, the tooltip should reposition to avoid clipping (for all plot types and in all relevant views)

A/Cs

• Hovering a point near the MultiLineChart's left/right edge shifts the tooltip
  box horizontally so it never renders past the plot area bounds
• Hovering a point near the MultiLineChart's's top/bottom edge shifts the tooltip
  box vertically (flips above/below the point) so it never renders
  past the plot area bounds
• Horizontal and vertical repositioning apply independently, so a point
  near a corner gets both adjustments at once
• The tooltip's pointer/tail always points at the exact hovered data
  point (no snapping to an axis) — unlike the stacked-area chart, this
  chart's anchor is a point, not the x-axis
• No visual change for points that already have enough space on
  all sides