Add string type to X on LineSeriesPoint

[TomStowe]

LineSeriesPoint should include the string type on the x-axis as this is permitted for all other types of graph

[EvgenyGulyaev]

If you want to download these types from NPM, I created 'react-vis-types' package
Please create PR to my Repo with your changes, thanks.
We can help other people

Install

npm install react-vis-types --save   
    or
yarn add react-vis-types

Repo

https://github.com/EvgenyGulyaev/types-react-vis

[EvgenyGulyaev]

@TomStowe ^