pro testplot
  ; Generate a thousand numbers from 0 to 2*pi:
  zero_to_2pi=indgen(1001)/500.0*!pi

  ; Calculate the sine of each of the list of numbers:
  sine=sin(zero_to_2pi)

  ; Tell imopen to open a portable network graphics ("png") file "test.png":
  imopen,'png',fn='test'

  ; The 'png' tells imopen the file format.  The fn='test' tells imopen the
  ; file's basename -- the portion of the filename that comes before the last '.'
  ; Later, when you call imclose, imclose will generate the file "test.png".
  ; It makes that filename by appending a "." and the name of the file format
  ; ("png") to the end of your file's basename ("test" from fn='test').

  ; Plot sin(x) with x=0..2*pi:
  plot,zero_to_2pi,sine

  ; Tell imclose to copy everything we plotted between imopen and
  ; imclose to the test.png file and close that file:
  imclose
end