The dialogue for the trend line creation/elimination is only available within desktop. Your report consumers will not have access to this option directly. You can however create two sheets, one with and one without the trend line and the parameter would allow the user to pick which sheet they might want to use: Creating a Sheet Selector for a Dashboard | Tableau Software . Further you could calculate the trend line yourself (requires some knowledge of table calculations above the average user and some patience). Once you created it yourself you could use it on a sheet and using Measure Names as a filter you could allow your end user to remove it at will, you could also once again use a parameter to swap between showing the value of the trend line that you calculated yourself and are now treating as a measure or making it null instead hence it would not appear on the chart. You can find information on the trend line creation bit here: https://interworks.co.uk/blog/linear-regression-trend-lines/
thanks a lot! Works great!