Have you looked at "Reference Lines" int he presentation tab of the chart, this allows you to create your own expression for the line so you should be able to create a trend line using that method.
Reference lines can only give you a vertical or horizontal line at variable positions, i am assuming he wants the possiblity to input some arbitrary equation, which i don't think is possible unless you want to make a separate expression for it which might work in some cases.
If the trend line is calculated according to other values already shown in the chart then why not add another expression, make it invisible and select one of the standard trend line options, this appears to work (I tried this one before replying).
