Hi Lukas,
We would like to let you know that currently, we don’t have support for hiding a trendline at the initial render and showing the legend entry. However, we have considered your requirement as a new feature and logged a feature request for it in our feedback portal.
We will prioritize the features of every release based on demand and priority. So, this feature will be available in any of our upcoming releases. You can also track the status of the feature with the feedback below.
Feedback, https://www.syncfusion.com/feedback/42718/add-support-to-synchronize-the-trendline-legend-toggle-state-with-respective
Regards,
Yuvaraj.