Gets the height of the box.
In some cases, the height of a box may be dynamically determined. An example is the height of an
LegendBox. For these cases, the height is undefined until the legend box or the entire chart has been laid out (using
BaseChart.layout or
BaseChart.layoutLegend), or the chart image has been drawn (eg. using
BaseChart.makeChart or
BaseChart.makeChart2).
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