This function will add a second x axis to a plot with the top ticks at arbitrary positions (you are not limited to the bottom tick positions). The new axis is created behind the original axis, and the two axes are linked together. This gives a much more intuitive behaviour, for example clicking in the figure window will select the original axis. It also alows you to drag the plot around, resize the limits and change between a log/linear scale without having to update the dummy axis manually.