Trendline on difference between two columns

[Old Red One]

How do I create a trendline illuminating the spread between two columns of
numbers, such as bond yields; e.g. a trend showing differencwe between a ten
year bond and ten year Treasury Inflation Protected bonds over time?

 

[Earl Kiosterud]

Red,

Make a third column in your sheet, include it in your chart, and change it
to a line chart. To change to line, when it comes up as a column, select
that series - right-click - Chart type - Line. The third column should have
a formula that calculates the difference.