Smarter chart

Hey,

I need some advice on a project I am about to start. Basically i found a template about forecasting in excel (See attached). On the dashboard tab you can select the dropdown box to display different dataset's and the chart will automatically update. I have tried to break the process down using all the cells but am only left more confused!!

Firstly how can you get data to change in set range of cells? Which formula is needed?
Secondly how can the process be simplified as it seems so confusing to trace formulas back and forward! I will only need this to calculate inbound calls against letter sent on a particular date.

Any help greatly appreciated.


Regards!