Press the Ctrl(Cmd on MAC) key on dragging and placing a panel. Or if you want to make all plot panels float, select 'Float' from Graph/Preperences in the menu bar. Otherwise choose 'Dock'.
Locate the mouse point on the plot to highlight and (Mouse) Right Click to bring up the full menu.
Locate the mouse point inside the graph which will be the center of the zooming, then rotate MouseWheel.
Locate (and click) the mouse point near x (or y) axis just outside of the graph and then rotate MouseWheel.
Note that this works only on the 1D plots.