You can drag either charge and find out the new electric distribution.
The field line are calculated in real time. However, please remember field line is not the same as trajectory for a test charge move in the same field.
You can add test charge if the check box were checked and watch the trajectory of test charge.
It is easy to draw those vectors which represent electric field at different positions.
Because we can calculate the electric field with .
However, do you know how those field lines were calculated?
Hint: The tangential components of electric field line is the same as the firection of electric field at the same point.