Legends at the bottom of graphs

### Version
5.0.2

### Reproduction link
[https://github.com/robertjorg/legend-overlap](https://github.com/robertjorg/legend-overlap)

### Steps to reproduce
I tried recreating with 5.2 but eCharts was giving me errors when trying to run. I was able to recreate the issue on the examples and have attached a github project displaying the issue.

Create a chart with a large legend section.

### What is expected?
The legend should move to the bottom of the graph and expand how much space is at the bottom of the graph.

### What is actually happening?
The legend will overlap with the actual graph when multiple lines in the legend are present. You are able to fix this by expanding the bottom size of the grid.

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We have a need to have dynamic legends. It would be nice if the legend would attach the top of the legend to the bottom of the graph and expand the bottom of the grid accordingly.