Adding the shift plugin as discussed on the mailing list today. It can shift forward/to the left (positive shift values) and backwards/to the right (negative shift values), and truncates the extra points so that the resulting vector has the same size as the input vector. Missing values are padded with NaNs, which achieves a nice effect: the curves really look like they are shifted (while padding with zeroes could be confusing in many cases). Finally, as suggested by Barth, the plugin is a filter, which means it can be called directly from a plot via the RMB/Filter menu. Simple, but cool :-)