Implement option to force event to work multiple xs type work as single x

[netil]

Description

When one of the below option is used, tooltip event will work only when mouse pointer is near to the data points:

• scatter, bubble type
• tooltip.grouped: false
• xs: { data1: "x1", data2: "x2" }

As single x event	As multiple xs event

Implement option to force multiple xs event type to work as single x event.

axis: {
    forceSingleX: true
}