Points can be selected only if the plot is active.
To take the mouse into select mode, choose Select from the Mode combo box or click on Select Points mode submenu from the Mode menu.
To select points, drag the left mouse button down and to the right to draw a box, the data points that fall within the box area are selected. A square with green outline is drawn around the data point to indicate that it has been selected.
For multiple selections of points, press the shift key and hold it while dragging the left mouse button.