Comparing weekly data from 2012 to the same data points for 2013, but on a daily basis

Hi all, I'm new to graphing, and while I thought this would be simple, I can't seem to figure it out.

I have weekly data from all of 2012 that I'd like to compare to the relevant 2013 year-to-date data, but that is daily instead of weekly. I'm looking to have a single graph, with two lines, one for the 2012 data extending longer towards the end of the graph, and one line for the 2013 data, ending shorter as there is only data for several months so far. They both measure the price performance of a stock index and the main goal is to see the 2013 performance so far compared to the same time period for 2012.

Is it possible to do this smoothly? Or should I have weekly data for both?

I have attached a sample spread sheet. For both data series, the left hand column is the date and the right column is the performance data measured in percentage.

Thank you!

I should mention that I would like the two lines layered on top of each other, rather than having one really long line.