pro plot_bias_calc,fna,fnb
;
common bias_calc,fa,fb,a,b,ha,hb
s=size(fa)
IF (s(1) EQ 0)  THEN fa=''
IF (fna NE fa) THEN BEGIN
   IF (fna EQ '')  THEN BEGIN
      PRINT,'ERROR: PLOT_BIAS_CALC, first file name not defined'
      RETURN
   ENDIF
   a = LASCO_READFITS(fna,ha)
   fa = fna
ENDIF
s=size(fb)
IF (s(1) EQ 0)  THEN fb=''
IF (fnb NE fb) THEN BEGIN
   IF (fnb EQ '')  THEN BEGIN
      PRINT,'ERROR: PLOT_BIAS_CALC, second file name not defined'
      RETURN
   ENDIF
   b = LASCO_READFITS(fnb,hb)
   fb = fnb
ENDIF
;!p.multi=[0,0,2,0,0]
ba =OFFSET_BIAS(ha,/sum)
bias_a = ba+findgen(15)
ma = average(a)
IF (ha.exptime LT 0)  THEN ha.exptime=ha.exptime+32
ma = (ma-bias_a)/ha.exptime
;plot,bias_a,ma,/yno,title=ha.date_obs+' '+ha.time_obs
bb =OFFSET_BIAS(hb,/sum)
bias_b = bb+findgen(15)
mb = average(b)
IF (hb.exptime LT 0)  THEN hb.exptime=hb.exptime+32
mb = (mb-bias_b)/hb.exptime
;plot,bias_b,mb,/yno,title=hb.date_obs+' '+hb.time_obs
!p.multi=''
xmin=min(bias_a)<min(bias_b)
xmax=max(bias_a)>max(bias_b)
ymin = min(ma)<min(mb)
ymax = max(ma)>max(mb)
plot,bias_a,ma,/yno,title=ha.date_obs+' '+ha.time_obs+' '+hb.time_obs ,$
   xrange=[xmin,xmax],yrange=[ymin,ymax]
oplot,bias_b,mb
RETURN
END