How to FIT

The chart is not going to do this for you. You can perform the "smoothing" analysis in the spreadsheet, and then add the smoothed signal to the chart easily enough. Here's how I would do it:

1) Perform regression. If you want to use the chart trendline, then copy the coefficients (make sure to increase decimal places to get sufficient precision) to the spreadsheet. For polynomial trendlines, I would prefer to use the LINEST() function (https://support.office.com/en-us/art...rs=en-US&ad=US which includes an example using a cubic polynomial) to perform the regression directly in the spreadsheet.
2) in column C, compute the regression value at each point. Either =a*a1^3+b*A1^2... or use the =SERIESSUM() function (https://support.office.com/en-us/art...4-96c49087f637 ).
3) In column D, perform the desired subtraction. I'm not familiar with this smoothing algorithm, so I'm not sure exactly what this subtraction should look like, but I will assume that you do.
4) Add column D to the chart as a new series. This can be done using copy->paste or the "select data" dialog.

What part do you have trouble with?