The Haar wavelet. Replacement for ASCII art:

           |
         1 ****O
           |    
           |   1/2 
   0  *****O---+---*****
           |        
           |        
        -1 |   ****O
           0    

See Wikipedia graph-making tips.

Generated in gnuplot with the following script:

 The source code of this SVG is invalid due to 4 errors.

 This W3C-invalid plot was created with gnuplot.

Then I opened the resulting SVG file in Inkscape, changed the fill style for the circles (either blue or white fill), line styles (to do dashed lines), brought the plot lines to the top, and then brought the circles to the top (over the lines).

Created by User:Omegatron using gnuplot, possibly with post-processing in the GIMP (PNG) or Inkscape (SVG)

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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License.http://www.gnu.org/copyleft/fdl.htmlGFDLGNU Free Documentation Licensetruetrue

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• Mathworld article