Color of the line connecting bubbles. The default value is the same as series' color.
In styled mode, the color can be defined by the
colorIndex option. Also, the series
color can be set with the .highcharts-series
,
.highcharts-color-{n}
, .highcharts-{type}-series
or
.highcharts-series-{n}
class, or individual classes given by the
className
option.