Smooth g Drag data points
g Y (ctrl-F8)
g X and Y (shift-ctrl-F8)
With this mode you can edit individual data points of a plot graphically. You pick the point up by clicking near it, and then you simply drag it to the place where you want it to be. You can drag several data points consecutively. As long as one plot is selected, clicking on the graph will select the nearest data point of the selected plot and you can start dragging it.
End this mode by right-clicking or by pressing <esc>.
Pressing ctrl-Z (undo) in this mode will undo the last point drag. Pressing ctrl-Z after ending this mode will undo all edits on all plots you have done during this mode.
You can use this mode to delete spikes (although the delete data points option is also very good for that purpose), or to perfect a graph you just drew yourself.