Double-click the left mouse button.
If you double-click in the plot area, all the zoom and pan operations on both the X- and Y-axes are reset and it returns to the original spectrum/chromatogram.
If you double-click in the area below the X-axis, all the zoom and pan operations on the X-axis are reset.
If you double-click in the area left of the Y-axis, all the zoom and pan operations on the Y-axis are reset.