plotOptions.mapbubble.lineColor

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.