Question

why does the graph of V/I always a straight line

Answer 1

because potential difference is directly proportional to current flowing through conductor

Answer 2

Ohm's law states that 
                          current (I)=Voltage (V) \ Resistance (R)
More specifically, Ohm's law states that the R in this relation is constant, independent of the current.
         so basically, a  simple resistor has a CONSTANT Resistance.When the current increases the voltage also increases and the resistance remains constant.   
          SO FINALLY THAT'S WHY THE GRAPH OF V\I IS ALWAYS A STRAIGHT  LINE.