To add a 3rd javascript library, I use to download its files and include them in my plugin.
The plugin.xml file will have an additional webResourceBundle dedicated to this library (in your case alertify).
Finally the webResourceBundle of the javascript view will include a webDependency tag for the added library webResourceBundle.
At a first sight, the source and relative paths attributes of webResource tag in your echarts’ webResourceBundle seem to be wrong: they should only include the path and not the filename.