If you want to see why, just look at the zero values.When . So the graph 'starts' at .
But when . So this graph 'starts' at , and it's when that we get the value .Now if we multiply the independent variable by , and replace with , we get the graphs of and . Now, in the second graph, we shall need to take to get our zero value of . So that's . So the graph has now been shifted to the left.
Finally, note that multiplying by , to give , makes no difference to the phase-shift: it merely 'stretches' the graph in a vertical direction either side of the x-axis by a factor of .