Pick
+ charge or
− charge in the toolbar, then:
Click/tap empty space → place a
±1 charge.
Click/tap a charge → change its charge by
±1.
Right-click (mouse) is a shortcut for
−1.
disclaimer: The simulation is not perfect, its purpose is purely illustrative and we take no responsibility for its accuracy. If it renders an unphysical field, try to increase the number of lines per charge, increase the number of steps or reduce the step size.
[code and docs]