Diagramming sentences is the only thing I remember from grammar instruction in Grammar School. Do native speakers even learn that anymore? How about in EFL classes? It's rare that I will even touch questions that go there, but on those rare occasions, a picture could say a thousand words - IF the pictures would be understood. Would they? If so, is there an easy way to do it online? If not, I can draw and scan, but I probably won't.
I suspect that what you're looking for is a parser which creates diagrams like this:
This is a Reed-Kellog diagram, and there is a parser here: http://1aiway.com/nlp4net/services/enparser/; but it’s not very satisfactory. Although it can handle fairly complex sentences with coordinate and subordinate clauses, it stumbles on simple matters like cleft sentences and locative complements.
Reed-Kellogg was still taught in US schools when I was a child, but disappeared when the teaching profession set its face against grammar of any sort in primary and secondary curricula. At about the same time, formal linguistics adopted tree diagrams of the sort snailplane's links provide; the link on Reed-Kellog describes these. To the best of my knowledge RK has never been widely known outside North America, though the form appears to be used by some teachers of ESL.
The question was raised last year on linguistics, and the comments there are interesting.
It is not common to use trees, but they can be used effectively. After all, if one needs to explain an entire sentence structure, then surely one can show a tree. And some people are visual learners. When needed, I use the following:
Miles Shang's Syntax Tree Generator (Can link to trees. This makes it nice to work with since you can recover the LBN.)
Yoichiro's RSyntax Tree (Best graphics result, but cannot link-to-tree 01/2014.)
They both use labeled bracket notation. You can make-up any syntax or categories you want. It's much easier to work with indented text.
Instead of working like this: [S [NP This] [VP [V is] [^NP a wug]]] Work with (and paste) this: [S [NP This] [VP [V is] [^NP a wug] ] ]
If the structure is complicated, I might start with Stanford Parser To give me a starting point. You end up with lots of parentheses and you have to search-and-replace-all to brackets. I use notepad and indent with spaces or tabs to keep everything straight (a programmer's editor would be better).
I'll use Stanford's LinGO English Resource Grammar (ERG) to give me other ideas, but it doesn't give LBN output.