hope this might be helpful for you.
used sample Superstore data file.
for Dimension Category got 3 items.
right click on category > create > set
set 1 name Category_Furniture for item Furniture
set 2 name Category_Technology for item Technology
now drag order date to column shelf ,sales to row shelf and set1 Category_Furniture to color on marks card
Line chart In value shows the furniture item sales and the Out value shows other 2 item sales as shown in below figure
Hide the Out value from the Line chart
drag linear trend line to view
now drop sales on to row shelf
drag set 2 Category_Technology to SUM(Sales)(2) color card and trend line as logarthmic, hide the out value
select dual axis
final output will be as shown in figure below.please check the attached workbook.
Trend_Line_Using_Sets.twbx 1.2 MB
To my knowledge you can't apply two different types of trendlines to subsets of dimensional data. What you can do is apply two different trendlines to two different measures on a dual axis graph, but that could get messy.