Chart Trend line help - should be easy!

[Singlespeed]

I have a line chart with 3 years worth of months on the X axis (36 points) and I have a 3 year trend line. what I need is another trend line for the last 12 months, I have added a field in my data with just the last months data points but when I add a trend line to that one it goes right back to the start when I need it just on the last 12 months!

Any ideas how I can fix it?