You can set a threshold for a pie chart which will group slices that are in value less than the threshold.

Here is the code on how to do this:

nav: {  
    "threshold":"5%",
    "others": {
        "text":"Others",
        "backgroundColor":"black"
    }
}

The others object sets the slice that includes all slices that are under the threshold level.

Here is a demo that shows how threshold works for pie charts: Demo

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