EDIT 130622===============
Finally found a trick for showing only the half top of the waveform automatically adjusting its height to the height of the panel. The problem is the plugin doesn't refreshes well, so you have to restart it clicking in another tab ("last.fm" for example) and in "Details" tab again.
Different polishes to maintain it usable using the minimum height for the panel
Download:
http://www.mediafire.com/download/todeqakoq2dlcks/Narrow_Waveform_130622.xml=======================
-No artworks, for people which have them in a sidebar
-Works well with dark and clear skins
-Possibility of also showing info about the artist or lyrics
(Old link removed having 75 downloads)
Note: The height of the bottom panel must be adjusted for seeing only the top half of the waveform as actually we can't set the height of the waveform as a calculation like "2 * Panel.Height - 15"