Automatically shift bottom legend down if the x-axis labels rotate / get longer

When positioning the legend at the bottom of the chart, the placement works nicely if tick labels take up only one line. But if the labels take up more vertical space, they will draw over the legend.

It would be nice if the legend could be pushed down automatically to account for the extra space required.

Corresponding codepen

As discussed before, let's revisit this in 6 - 8 weeks time when hopefully there will be more capacity for this complex issue.

先从对应的 CodePen 开始，重现旋转的或多行 x-axis 标签与底部图例之间的重叠，然后追踪 plotly.js 中图表布局和图例定位的入口点。完成的标准是：只要标签需要额外的垂直空间，图例就会自动向下移动，且不会发生重叠。