title: How plot on Images with Plots.jl?
body: <p>I want using Plots.jl for plot on Images, for example simple sinusoid. Here my code: </p>
  
  <pre><code>using Plots
  using Images
  gr()
  h = 400
  w = 600
  a = Array(RGB{FixedPointNumbers.UFixed{UInt8,8}}, h, w)
  img = Image(a)
  p=plot(img)
  x = collect(0:0.1:2π)
  plot!(x,sin(x))
  png("Test")
  </code></pre>
  
  <p>But I get wrong result. How do this correctly? </p>