docs: def __zoom(self,event,scale=2.0):
          """Zoom in"""
  
          import Tkinter,math
          ## compute the x,y of the center of the screen
          sx1=self.cx1+(self.cx2-self.cx1+1.0)/2.0
          sy1=self.cy1+(self.cy2-self.cy1+1.0)/2.0
  	#print sx1,sy1
  	if not event is None:
  		sx1=event.x
  		sy1=event.y
  	#print sx1,sy1
          ## translate that into a canvas location and then
          ## and ra/dec position
          (x,y)=self.c2p((self.canvasx(sx1),self.canvasy(sy1)))
  	#print math.degrees(x),math.degrees(y),sx1, sy1
          ## reset the width of the display
          xw=(self.x2-self.x1)/2.0/scale
          yw=(self.y2-self.y1)/2.0/scale
  
          ## reset the limits to be centered at x,y with
          ## area of xw*2,y2*2
          self.limits(x-xw,x+xw,y-yw,y+yw)
  
          self.delete(Tkinter.ALL)
          doplot(kbos)