Allow to set the `series-graph.lineStyle.[width|color]` by a function

### What problem does this feature solve?

So I'm able to set the width dynamically without changing the whole option structure to behave more dynamically.

### What does the proposed API look like?

`series-graph.lineStyle.[width|color] = () => 5`
`series-graph.emphasis.lineStyle.[width|color] = () => 5`