If you want to have item in legend for group of series and also save in legend every series of the group, you could do like this:
{
name: 'Owners',
id: 'Owners'
} as SeriesOptionsType,
{
name: 'Owners, 2010',
data: [15542, 13564, 15654],
linkedTo: 'Owners',
showInLegend: true
} as SeriesOptionsType,
{
name: 'Owners, 2011',
data: [4532435, 342534, 65776],
linkedTo: 'Owners',
showInLegend: true
} as SeriesOptionsType,

LinkedTo boundes all owner series. showInLegend saves every owner series item in legend

I would like to group these three series:

Ping5 - Avg
Ping5 - Max
Ping5 - Min

It should be grouped like
Ping5
- Avg
- Max
- Min

(4 lines)

or maybe

Ping5 - Avg - Max - Min

(1 line)

Clicking on "Ping5" will hide all three Series and clicking on "Avg" will only hide one.

Grouping of series would be nice yes, as described in the topic. For the time being, I guess one needs to put a link somewhere, that does this using javascript.

I'm currently plotting data for N-Groups, and each group has 3 series of data on the chart. I want to display tool tips per group, along the x-axis instead of all points on the x-axis.

That's what I'm missing on it. I've created 3 pies in a same chart but there is no way to use just one legend for all of them.