Tooltip option to only show overlapping series in shared tooltip.
The tooltip.shared: true option currently shows the tooltip for all series.
However for charts with many series this is becoming a lot of information. Therefore it would be very handy to add a option which only shows the lines which overlap in the tooltip.
You can see this as a sample for two lines here: https://stackoverflow.com/questions/18988329/highcharts-use-shared-tooltip-only-when-series-overlap