A plot can be zoomed only if the plot window is active.
For zooming into the plot you need to be in zoom mode. To go in zoom mode, choose zoom from the Mode combo box or click on the Zoom Mode submenu from the Mode menu.
If the number of points for the region zoomed in is greater than 100000, pixels are used for plotting.
To zoom in, drag the left mouse button down and to the right to draw a box around an area that you want to see in detail.
To zoom out, drag the left mouse button up and to the left.
To get the original graph click on the "Show Full Range" icon represented by 