Is there any way to set the data range of the trend line specifically without changing the entire data range of the original plot?
Correct me if I am wrong, but you I suppose you use the graph-function to insert a trend-line. You can also use the TREND function to calculate a trend-line in your own interval:
=TREND(A19:A38;B19:B38)
(this is a matrix-formula, so you have to use ctrl+shift+enter)
and then incorporate the line in your graph.
Jens