what is chart.js , when you would use it. Next,

what is chart.js , when you would use it.  Next, find a website that uses the method or should use the method. Explain why ( explain about ecosystem, opensource javascript library ease of use, strong developer community, explain about plugins mainly annotation and zoom plugin etc)

References
Websites that use chart.js 

https://www.wappalyzer.com/technologies/javascript-graphics/chart-js