The voltage at point D = V × R0/ (R + R0)
The voltage at point C = V × R / (R + R)
The voltage (V) across galvanometer or between C and D is,
VCD = V R/ (R + R) - V × R0/ (R + R0)
R0 is change from 0 to infinity.
when R0=0 VCD is positive. so graph should be start in a positive value.
when 0<R0<R VCD is positive. I positive
when R0=R VCD=0 so I =0
when R<R0<infinity VCD is negative.I negative (direction change)
here R<<<infinity . so graph cross the Ro axis more close to the zero than infinity.