g2@4.2.4 can't show legend

[chi-gao]

• I have searched the issues of this repository and believe that this is not a duplicate.

Reproduction link

Steps to reproduce

as the code shows, I set the legend visible, but it doesn't show.
I had set the line size, line color and line shape attribute.
and I had tried followings:

• chart.legend(true)
• chart.legend({ position: 'bottom'})
• add bottom padding

Environment	Info
g2	4.2.4
System	-
Browser	-

[chi-gao]

尝试g2降级到3.x，上述写法 图例是完美展示的。
目前可以确定确实是升级4.x带来的问题，所以不清楚是 4.x底层默认不支持 双轴图的图例展示， 还是 4.x升级时这部分逻辑漏掉了。
如果设计不支持，建议升级文档能够说明下，毕竟也算是 break change 了。

补充一个3.x的demo：https://codesandbox.io/s/young-mountain-x1u73c