Added new plot library and old libfit (which plot still depends on)

original commit: d3b5e21cae5deb672a921a05c3bad15db1545ef1

collects/mrlib/scribblings/plot.scrbl

@@ -6,6 +6,10 @@
 @defmodule[mrlib/plot]{The @racketmodname[mrlib/plot] library provides
 a simple tool for plotting data values to a device context.}
 
+@bold{This is an old library, kept only for compatibility.} You will
+undoubtedly want to use the @racketmodname[plot] library instead, which offers
+many more features and is actively maintained.
+
 @defstruct[data-set ([points (listof (is-a?/c point%))]
                      [connected? any/c]
                      [pen (is-a?/c pen%)]