Suggested update to example 20 to demonstrate distorted plotting of an  image more clearly. This change also illustrates how to use pltr_data  to pass information to pltr functions.
Suggested update to example 20 to demonstrate distorted plotting of an 
image more clearly. This change also illustrates how to use pltr_data 
to pass information to pltr functions.


svn path=/trunk/; revision=8546

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot