Multiple trendlines and automating changes

[Martini101]

I have a set of outputs each month that I have plotted into a graph to show the output, the upper and lower capability, and the average expected output per month. I want to create an automated spreadsheet which can adjust the upper and lower levels and the average per month based on 2 months of consistently increasing/decreasing output. I know I can manually plot the data in, and put in multiple series to show on the graph, but im really struggling with making the sheet do this automatically, without me having to adjust the series every time there is an improvement / decrease in activity. I'm fine with using formulas and formatting etc but have a suspicion that this will require some visual basic, and im a complete novice at that! Can anyone help out?