Props should not be mutated by Plot component

It seems like the Plot component mutates its props (at least layout) instead of cloning the data.
Mutation of props is to me unexpected (and undesired) behavior for a react component.

If for instance the layout is stored in the state of a component and then fed to the Plot component as a prop, like

render() {
  <Plot
    layout={this.state.layout},
    ...
    />

we will have a problem, since this.state only should be updated with the react setState() method.

此 issue 指出了 Plot 元件及其 layout prop；請先找出元件對 layout 的處理方式，並使用由 state 保存的 layout 重現該行為。當元件不再修改傳入的 layout，並透過回歸測試完成驗證時，即表示完成。