Chart to show trendlines comparing year over year data

Hello again!

My first inquiry was answered so quickly and accurately I thought I'd try again!!

I have a chart where I want to show two trendlines that compare year-over-year data. 

The chart has 'On the change of' @Month (where the formula for MONTH is....DatePart ("m",{PTS_1_18_13_10_09.Entry Date/Time})  to 'Show Values' for 2 running totals: #2013 and #2014.  So far so good.  The issue is that for the months where there is yet to be data (Nov 2014 and Dec 2014), the 2014 trendline drops down to 0, like this:

          |  /\

          | /  \/\             

2014  |/       \/\          /\     

          |  /\        \    /\/   \

2013  |/   \     /\ \ /        \  

          |      \  /   \/\

          |____\/____ \_______________  (Monthly points graphed)

          Sept  Oct   Nov    Dec

Hopefully this is understandable .

How can I just have the 2014 trendline discontinue where the data stops (Oct 2014) instead of it dropping down to 0?

Ideally it would like this:

          |  /\

          | /  \/\             

2014  |/       \/\          /\     

          |  /\             /\/   \

2013  |/   \     /\   /        \  

          |      \  /   \/

          |____\/____ _______________  (Monthly points graphed)

          Sept  Oct   Nov    Dec

Thanks so much!