There is a +Q point charge on the x-axis at x = -2, and a -3Q point charge on the x-axis at x = +2. Taking positive to mean a field pointing to the right, which graph correctly shows the electric field as a function of position on the x-axis?

1. Graph 1 (in black)
2. Graph 2 (in red)
3. Graph 3 (in green)
4. Graph 4 (in blue)
5. Graph 5 (in purple)

The correct one is graph 4, the blue one.
In the region between the charges the field goes to the right so the graph is positive, and goes off to positive infinity as you approach either charge.
In the region x > +2 the field is to the left so the graph is negative, going to negative infinity near the -3Q charge and then approaching zero as x increases.
In the region x < -2 the field is to the left near the +Q charge so the graph is negative there. Although you can't really see this on the graph the field passes through zero at some point in that region, and beyond that is positive as the -3Q charge dominates far from the charges.