Trendline - Which one to use

Hi all,

I am currently doing some graphs on previous figures over periods. I am trying to add a trend line in however I am not sure which one fits best.

i have attached a pic of the graph I have created with a linear trend line in, but if you can shed any light on which trend is best for this type of data / what they all mean?

Many thanks,

Jon

http://img26.imageshack.us/i/graphc.jpg/