Y Axis and line graphs
 

I have two questions both regarding the 'y' axis on a line graph
Firstly how do i get the 'y' axis to appear at the center of the graph IE at
(0,0) and not (-10,0) as it is currentley on my graph
Secondly how would i plot the line X = 3 which should just be a straight line

An unrelated question but is it possible to graph functions in excel? right
now i just put the x values in one column and calculate the y values in
another and copy the formula down - is there a quicker way?