🐛 [BUG] 折线图canvas模式折线下边沿的label不显示

### 🐛 Bug description [Please make everyone to understand it]
折线图canvas模式下线下label不显示，并且SVG和canvas下表现不一致（当图例的容器宽度比较小，label的文本多，部分label会往挤到下面，这个时候会触发）
canvas下
svg下( svg下展示了所有数据，我觉得这个因该是正确的)

### 💻 Link to minimal reproduction
https://codesandbox.io/s/stupefied-forest-c5dqm
### 📷 Step to reproduce
### 🏞 Expected result
应该和svg下表现一致
### 🚑 Any additional [like screenshots]

* **G2Plot Version**:
* **Platform**:

Start with the linked CodeSandbox reproduction and compare the line-chart label layout in canvas and SVG modes, especially when the legend container is narrow and labels wrap. Done means the canvas rendering displays the same lower-edge labels as SVG in that case.

Written by the indexing model from the issue text.