Erase all graphics from the current PGPLOT device.

CONTEXT
-------
Note that to erase all graphics, this command actually performs an
area fill using the current color of color index 0. One use of this
function is to set the background color of a plot before plotting
any graphics. This is basically the only way to set the background
color of a postscript plot. For example:

0>dev /ps
0>pgscr 0, 0.0, 0.0, 1.0
0>pgeras
0>box 0,1,0,1
0>pgend
0>$ghostview pgplot.ps
0>

The above starts by redefining color index 0 to be blue, then uses
pgeras to fill the background, then plots axes on the plot and finally
after closing the postscript file, plots the file with ghostview.