Do not attempt to plot graphs when the actual request for bandwidth data failed

The console log reported in the blog comment was

17:00:04.152 TypeError: d3.select(...).node(...) is null
mainDetailsView<.plot() main.js:169
mainDetailsView<.render/<.success/<() main.js:220
forEach() self-hosted:208
b.forEach() underscore-min.js:11
mainDetailsView<.render/<.success() main.js:210
graphModel<.lookup_weights/xhr<() graph.js:150
f.Callbacks/n() jquery-min.js:2
f.Callbacks/o.fireWith() jquery-min.js:2
w() jquery-min.js:4
.send/d() jquery-min.js:4
1 main.js:169:20

I've not verified that this is reproducible yet.