ref line on $mark mouseover

[NicolaLady]

• $mark .on(mouseover, drawLine())
• drawLine() --> append line --> x1, x2, y1,y2
• drawing from y1 --> y2( cy) pos of currently selected mark
  -drawing from x1 --> x2 (cx) pos of currently selected mark
• text is a matter of grabbing corresponding elem in array for selectedTimePoint
• gdp[country,selectedTimePoint]
• child[country,selectedTimePoint]
• use same selections to update ref line on $mark mouseover #10 tooltip styling