Added plot recipe

Author: hotplot

src/PlotRecipes.jl

@@ -0,0 +1,10 @@
+using RecipesBase
+
+@recipe function poly_recipe(p::Poly, xs = polyinterval(p))
+    buff = IOBuffer()
+    printpoly(buff, p)
+    label --> String(take!(buff))
+
+    ys = map(x -> polyval(p, x), xs)
+    xs, ys
+end

src/Polynomials.jl

@@ -5,7 +5,7 @@ module Polynomials
 
 export Poly, poly
 export degree, coeffs, variable, printpoly
-export polyval, polyint, polyder, roots, polyfit
+export polyval, polyint, polyder, roots, polyfit, polyinterval
 export Pade, padeval
 
 import Base: length, size, eltype, collect, eachindex, lastindex
@@ -716,7 +716,43 @@ function polyfit(x, y, n::Int=length(x)-1, sym::Symbol=:x)
 end
 polyfit(x,y,sym::Symbol) = polyfit(x,y,length(x)-1, sym)
 
+
+"""
+    polyinterval(p)
+
+Choose a sensible interval containing points of interest for the polynomial `p`.
+
+The interval contains some margin either side to allow for plotting.
+
+# Examples
+
+```julia-repl
+julia> polyinterval(Poly([0, 0, 1])) # contains single critical point at x=0
+-1.0:0.01:1.0
+
+julia> polyinterval(Poly([1, 0, 3, -1])) # points of interest between 0 and 3.1
+-3.103803402735535:0.031038034027355346:6.20760680547107
+```
+"""
+function polyinterval(p :: Poly)
+    # Find points of interest
+    zero_pts = roots(p)
+    crit_pts = roots(polyder(p, 1))
+    infl_pts = roots(polyder(p, 2))
+    pts = sort([ real(pt) for pt in [zero_pts; crit_pts; infl_pts] if isreal(pt) ])
+
+    # Choose a range that shows all interesting points with some margin
+    min_x, max_x = length(pts) > 0 ? (pts[1], pts[end]) : (-1, 1)
+    diff = max(max_x - min_x, 1)
+    a = min_x - diff
+    b = max_x + diff
+
+    return a : diff/100 : b
+end
+
+
 ### Pull in others
 include("pade.jl")
+include("PlotRecipes.jl")
 
 end # module Poly