for starters, I'm not too sure why do you need to use AGGR() in your calculation. If both Day/Month/Year and FN2 are your chart dimensions, then AGGR() is redundant. If they are not, then you need to apply another aggregation function, such as sum(), on top of the AGGR().
With regards to your questions:
- You add another Line to the line Chart by adding another Expression.
- In order to calculate differences in quantities between the "current" value, the previous value and/or the first (or last) value in the chart, you can use functions Above(), Below(), Top(), and Bottom(). Check out the Help Article "Chart Inter Record Functions".