To create a trendlines in Excel 2016 app, we suggest that you check this
article for a step-by-step instruction. Excel Online runs on limited features, and as of the moment, we’ll have to wait until it’s designed to support trendlines. Of course, we don't want this idea to go to waste. As such, we strongly believe that this
needs to be heard in our
Excel Online UserVoice feedback platform. This way, you can help us improve Excel Online’s features and overall functionality in future updates and for our Microsoft Engineers has better visibility on what features are most important to you.
Let us know if you need other concerns that we can assist you with.
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