invert/ flip data series

Hi,

I'm new to this forum so I'm looking forward to learning how to use Excel a lot more effectively

I'm working with a variety of motion capture data like the rotation of a leg from the start to the end of a walk cycle. Plotting a number of these different mocap walks as leg rotations shows peaks and troughs, however sometimes I have had to use data starting on the opposite leg. To better compare the data series against the others how can I:

1. flip or invert the data series, assuming the axis is 0? (changing positive values to negative and vice versa?)
   
2. calculate the central axis between the points (like a trendline?), and then invert the values around that, if the central axis is not 0?

I need to flip the data on the red line shown below:
16i8nsn.jpg

Thanks