escape graph_name before pass to rrdtool

We can register any name for graph_name, for example "Hatena::Blog::Graph".  But rrdtool use colon for delimiter, so we cannot view graph if graph_name includes colon.

So I fix to escape ":" by "\:" before pass graph_name to rrdtool.

See also
- http://oss.oetiker.ch/rrdtool/doc/rrdgraph_graph.en.html