NAME:
	mk_plot_diff
 PURPOSE:
       this program makes the plots that diffmag_wfc inserts into ps or gif 
	files: log(N) vs mag where N=histogram(mag)
 CALLING SEQUENCE:
	mk_plot_diff,mag,char[,class=class,ccd=ccd]
 INPUTS:
	mag: array containing magnitudes to plot histograms
	char: character size to use
 OPTIONAL OUTPUT KEYOWRDS:
	class: STRING! used to make legend for the graph: 
		'stars'
	if class not set then get: 'All objects'
	ccd: STRING! used to make title 'ccd1' 
	if ccd not set then just get 'All stars' or 'All objects'
 EXAMPLE:
	mk_plot_diff,stars1,0.75[,class='stars', ccd='1']
 CALLED PROCEDURES:
	astrolib
 MODIFICATION HISTORY:
	2-Nov-2000  Original by Suzanne Aigrain

(See /home/xmmssc/soft/idl/pro/wfcqc/mk_plot_diff.pro)