Trendline in Barchart

Short but useless answer -- yes, it is possible to add a trendline to a 2D clustered bar chart. According to the help file linked below, stacked bar charts do not accept trendlines, so it might depend on exactly what kind of bar chart you have. I would expect:
1) Data arranged in spreadsheet so that you can easily create a clustered bar chart with multiple data series (I assume you can already do this).
2) Create bar chart.
3) Add trendline to the desired data series: https://support.office.com/en-us/art...rs=en-US&ad=US
4) Other formatting as desired.

What step are you having trouble with?