here we have x and y axis, y axis represent height and x axis represent number of days
it is plotted in the manner,that if x is 1 then y is 2.if x is 2 then y is 4. so the height is constantly increasing with 2.so we can conclude that.f(x)=x*2.so the given graph represents a not function.Because when we consider x=3.it must show 6,but its not 6 in there.so when number increses its not following f(x)=x*2.so its wont make function.