I have a line graph that plots a series of data for months Jan-Nov. I only
want to add a trendline to the second half of the data on the graph. Example,
I only want a trendline from June-Nov, but I want the whole Jan-Nov series
plotted as one line on the graph.

How do I add a trendline for only half of the data? Do I need to create 2
series on the graph? And if so, how do I do this in a way that both series
are as one continuous line on the graph?