NAME:
	oplot_diff
 PURPOSE:
       this program overplots log(N) vs mag where N=histogram(mag) over an 
	existing plot with specified linestyle
 CALLING SEQUENCE:
	mk_plot_diff,mag,char[,class=class,ccd=ccd]
 INPUTS:
	mag: array containing magnitudes to plot histograms
 OPTIONAL OUTPUT KEYOWRDS:
	class: STRING! used to make legend:'stars',... 
	if class not set then get: 'all'
	linestyle: determines what linestyl to use when plotting
		default is 1
 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/oplot_diff.pro)