abstract PlotRecipe

getRecipeXY(recipe::PlotRecipe) = Float64[], Float64[]
getRecipeArgs(recipe::PlotRecipe) = ()

plot(recipe::PlotRecipe, args...; kw...) = plot(getRecipeXY(recipe)..., args...; getRecipeArgs(recipe)..., kw...)
plot!(recipe::PlotRecipe, args...; kw...) = plot!(getRecipeXY(recipe)..., args...; getRecipeArgs(recipe)..., kw...)
plot!(plt::Plot, recipe::PlotRecipe, args...; kw...) = plot!(getRecipeXY(recipe)..., args...; getRecipeArgs(recipe)..., kw...)


# -------------------------------------------------

function rotate(x::Real, y::Real, θ::Real; center = (0,0))
  cx = x - center[1]
  cy = y - center[2]
  xrot = cx * cos(θ) - cy * sin(θ)
  yrot = cy * cos(θ) + cx * sin(θ)
  xrot + center[1], yrot + center[2]
end

# -------------------------------------------------

type EllipseRecipe <: PlotRecipe
  w::Float64
  h::Float64
  x::Float64
  y::Float64
  θ::Float64
end
EllipseRecipe(w,h,x,y) = EllipseRecipe(w,h,x,y,0)

# return x,y coords of a rotated ellipse, centered at the origin
function rotatedEllipse(w, h, x, y, θ, rotθ)
  # # coord before rotation
  xpre = w * cos(θ)
  ypre = h * sin(θ)

  # rotate and translate
  r = rotate(xpre, ypre, rotθ)
  x + r[1], y + r[2]
end

function getRecipeXY(ep::EllipseRecipe)
  x, y = unzip([rotatedEllipse(ep.w, ep.h, ep.x, ep.y, u, ep.θ) for u in linspace(0,2π,100)])
  top = rotate(0, ep.h, ep.θ)
  right = rotate(ep.w, 0, ep.θ)
  linex = Float64[top[1], 0, right[1]] + ep.x
  liney = Float64[top[2], 0, right[2]] + ep.y
  Any[x, linex], Any[y, liney]
end

function getRecipeArgs(ep::EllipseRecipe)
  [(:line, (3, [:dot :solid], [:red :blue], :path))]
end

# -------------------------------------------------


"Do a correlation plot"
function corrplot{T<:Real,S<:Real}(mat::AMat{T}, corrmat::AMat{S};
                                   colors = :redsblues)
  m = size(mat,2)

  # might be a mistake? 
  @assert m <= 10
  @assert size(corrmat) == (m,m)

  # create a subplot grid, and a gradient from -1 to 1
  p = subplot(zeros(1,m^2), n=m^2, link=true)
  cgrad = ColorGradient(:redsblues,[-1,1])

  # make all the plots
  for i in 1:m
    for j in 1:m
      idx = p.layout[i,j]
      if i==j
        # histogram on diagonal
        plt = histogram(mat[:,i], c=:black)
      else
        # scatter plots off-diagonal, color determined by correlation
        plt = scatter(mat[:,j], mat[:,i], ms=4, c=getColorZ(cgrad, corrmat[i,j]))
      end
      # replace the plt
      p.plts[idx] = plt
    end
  end

  # link the axes
  subplot!(p, link=true)
end