Attempt to make HARP section of histogram clearer

Now has thicker hatching and a pink background in the overlap region.
Since the epslatex driver can't do transparency, it had to be
done manually with an extra plot series.
Darken background slightly

(After a test printout on the Epson plotter.)
Add a faint border to the poster

This is supposed to match the lines in the background and
should make cutting it out easier!
Slightly increase header area height

Since we had a little room at the lower edge.
Be less specific about the island S/N ratio

Requested by SFG.
Update the plots

The color scales have been tweaked as the previous version
looked too dark printed on the Epson plotter.  Also added a white
outline to the peak markers to make them stand out more.
Update gfx directory .gitignore file