One-line summary [problem brief] 
 how to switch between topics, is there any way? 
 Version & Environment [version and environment] 
 ECharts version [ECharts version]: 4.1.0 
 Browser version [browser type and version]: Google 67.0.3396.87 (official version) 
 OS Version [operating system type and version]: Microsoft win 10 
 Expected behaviour [expected result] 
 switch topics by method 
I can pass in themes when I find init, but how to switch themes after initialization
you can see
in http://gallery.echartsjs.com/.. 
 
you can click directly to switch topics, but there is no relevant method found in the document
is there any other documentation related to echarts topic configuration?

