Web Structure only showing one node for 400 urls...?

I’m trying to get to know Yacy to use it as a topic-oriented search portal.
As a first attempt I pasted 400 host urls into the Start Point box in Expert Crawl Start and then selected options to a Crawl Depth of 2 and set the Load Filter on URLs to ‘restrict to start domains’.
I started the crawl and six hours later I have about 56,000 documents in the index.
Which is great !

I am just wondering about the options on the Web Visualisation page… becayse the Image Collage shows nothing and the Web Structure is just displaying a single node.

I have done this before, adding one Url at a time and got a really cool graph showing connections between sites… but not this time…

So I’m wondering what I might have done wrong?

Thanks in advance.