spacingBottom is what you need to add, and also you need to take care of the offset:

chart: {
            events: {
                load: function () {
                    var label = this.renderer.label("How do I move this center and under the legend.")
                        width: '450px',
                        color: '#222',
                        fontSize: '16px'
                        'stroke': 'silver',
                        'stroke-width': 2,
                        'r': 5,
                        'padding': 10

                    label.align(Highcharts.extend(label.getBBox(), {
                        align: 'center',
                        x: 0, // offset
                        verticalAlign: 'bottom',
                        y: 50 // set the right offset
                    }), null, 'spacingBox');

            renderTo: 'container_chartFreqAtaTailNum',  
            type: 'bar',
            height: 895,
            spacingBottom: 100 // The space between the bottom edge of the chart and the content (plot area, axis title and labels, title, subtitle or legend in top position).         

Here is the fiddle:

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