add simple tooltips for a line that's based on d3

tooltips-for-d3-line.coffee

    svg.selectAll('.views_dot')
        .data(data)
        .enter()
        .append('circle')
        .attr("transform", "translate(10, 0)")
        .attr('class', 'views_dot')
        .attr('fill-opacity', 0)
        .attr('cx', (d)->
          x( new Date(d.date) )
        )
        .attr('cy', (d)-> y(d.value))
        .attr('fill', 'orange')
        .attr('r', 5)
        .attr('data-toggle', 'popover')
        .attr('data-content', (d)=>"#{d.date}: #{@formatValue()(d.value)}")
        
  
  # tooptip for d3 line chart
  $('.views_dot').popover
      title: ''
      container : 'body'
      html      : true
      trigger  : 'hover'
      placement : 'top'