Hi Mark,
Quite often we test some trendlines, to see if they may be relevant or not at a certain position on the graph, but don't keep them. It seems that currently it is not possible to cancel the drawing of a trendline. We have to draw it and then to delete it.
Suggestion: when the drawing process is ongoing (ie. Ctrl+Shift+ leave left button pressed and move the mouse to adjust the trendline), be able to cancel the trendline in progress with right click (so, when the left mouse button is still pressed, press the right button once to cancel the current trendline).
Be able to use the Esc key (if preferred instead of the right click) to cancel the same way with the keyboard would be nice too.
This is a quite standard feature in drawing/charting programs.
PS: Currently, when a right click is done during the drawing, the contextual menu appears and that creates a ghost trendline (it becomes white and cannot be modified, we have to refresh the graph to make it disappear), it seems it's a bug.
Thanks