is it possible to adjust convex hull to draw blue line instead of green line?

[meInvent bbird]

i succeed to use code to draw green line, but green line not draw the 
large area, expect second uploaded picture, the blue line connect
the bottom of red line graph

im = img.copy()
cntcounter = 0
for cnt in contours:
        #epsilon = 0.1*cv2.arcLength(cnt,True)
        #approx = cv2.approxPolyDP(cnt,epsilon,True)	
        #peri = cv2.arcLength(cnt, True)
        #approx = cv2.approxPolyDP(cnt, 0.15 * peri, True)
        #print("len(approx)="+str(len(approx)))
        #if len(approx) == 2:
            print("approx=" + str(approx))
            cntcounter = cntcounter + 1
            print("here1")
            #x,y,w,h = cv2.boundingRect(cnt)
            hull = cv2.convexHull(cnt,returnPoints = True)
            print("here2")
            while im is None:
                time.sleep(1)
            if im is not None:                
                print("here3")
                #cv2.rectangle(im, (x,y), (x+w, y+h), (0,255,0), 2) 
                #cv2.imwrite("C:\\Users\\tester\\Documents\\masda"+str(cntcounter)+".png",imi)                
                #im = cv2.line(im,(x,y),(x+w,y),(0,255,0),2)
                #im = cv2.line(im,(x+w,y),(x+w,y+h),(0,255,0),2)
                #im = cv2.line(im,(x,y+h),(x+w,y+h),(0,255,0),2)
                #im = cv2.line(im,(x,y),(x,y+h),(0,255,0),2)
                previousx = 0
                previousy = 0         
                for c in hull: 
		    if previousx != 0 and previousy != 0 and c[0][0] != 0 and c[0][1] != 0 and abs(previousy - c[0][1]) > 10 and abs(c[0][0] - previousx) > 1:    
                        while im is None:
                            time.sleep(1)
			cv2.line(im,(previousx,previousy),(c[0][0],c[0][1]),(0,255,0),2)
                        print("")
			#print("previousx=" + str(previousx))
			#print("previousy=" + str(previousy))
			#print("c[0][0]=" + str(c[0][0]))
			#print("c[0][1]=" + str(c[0][1]))
		    previousx = c[0][0]
		    previousy = c[0][1]



https://drive.google.com/file/d/0Bxs_ao6uuBDUWVBFZzVIVGotRlk/view?usp=sharing

expected is

https://drive.google.com/file/d/0Bxs_ao6uuBDUNGZFS2F3WnJERzA/view?usp=sharing