Added method option to build_slopegraph
Added the 'none' method to build_slopegraph
Added the 'rank' method
First draft of Tufte sort.  Doesn't work yet
Got the Tufte algorithm working, though it broke the plotting
Tidied up the code
Tidied up plotting theme
Added a fix to ensure there's a valid point defined for every value
Allow users to do their own digit rounding