Unfortunately, these sources are not available.
DEA's visualization support uses yFiles which is a commercial product,
therefore it's not available in the Community Edition. In the Ultimate
you can use yFiles in your plugins by means of IDEA's wrapper classes
com.intellij.openapi.graph package). There's no documentation for it, but
it essentially mirrors the yFiles API.
We're no longer publishing javadocs as a separate download: you can
refer to the Community Edition source for any necessary information.