How to use the @nivo/bar.BarDefaultProps.indexBy function in @nivo/bar

required: true,
        group: 'Base',
    },
    {
        key: 'indexBy',
        help: 'Key to use to index the data.',
        description: `
            Key to use to index the data,
            this key must exist in each data item.
            
            You can also provide a function which will
            receive the data item and must return the desired index.
        `,
        type: 'string | Function',
        required: false,
        defaultValue: defaults.indexBy,
        group: 'Base',
    },
    {
        key: 'keys',
        help: 'Keys to use to determine each serie.',
        type: 'string[]',
        required: false,
        defaultValue: defaults.keys,
        group: 'Base',
    },
    {
        key: 'groupMode',
        help: `How to group bars.`,
        type: 'string',
        required: false,
        defaultValue: defaults.groupMode,