Merge pull request #92 from ijlyttle/master

fixes issue #91 on documentation
Hacks the Minard case study

* I don't know if you mean to refer to the HistData package, but I put in a reference (and a suggests, which may be overkill).
* I played with the ggvis example until (I think) it works. In the process, I may have wrecked your point of how to make more clear, vs. ggplot2, the data-to-mark mapping. Is the `node()` function something else now?
* Please feel free to refuse this pull request :)