Block change when source changes
 

I am wanting to trend and graph some data on a month by month basis over the
year. My problem is that the source is a single data cell which is being
updated on a weekly basis. I need to be able to read the data for a month
(say Jan) into a cell in the graph worksheet which is allocated to that
current month (Jan), and then when time moves on to the next month (Feb) I
need the destination cell for Jan to hold its value, and the source cell
value for the new month to go into the Fe destination cell, and so on. Any
ideas on how to lock the value in the past month and move onto the next
month, all from the common source cell???