Incorrect hover label for missing last value with hovermode `x-unified` and `tickson=boundaries`

In this codepen the hover label for the last x axis value is incorrect. It displays "AC" in the hover, even though the correct value should be "Other".

The problem seems to be related to the missing last value (null) in the line trace. If that value is present, the hover labels work correctly.

The problem disappears when changing the width of the chart or reducing the number of data points.

从链接的 CodePen 复现开始，检查 hovermode x-unified 配合 tickson=boundaries 时的悬停标签行为，重点关注最终值为 null 的 line trace。在不同的图表宽度和数据点数量下验证结果；当最后一个值缺失时，最终悬停标签显示“Other”而不是“AC”，即表示完成。