TWO Point Charges on the x-axis and an electric field graph

[danielcheung1]

Homework Statement

Two point charges qA and qB are located on the x-axis at x=a and x=b. The figure is a graph of Ex, the x-component of the electric field.
What are the signs of qA and qB?

Homework Equations

http://session.masteringphysics.com/problemAsset/1071455/4/29.EX31.jpg

The Attempt at a Solution

I feel like... since the graph is symmetrical, the two charges should be the same sign, eg, either both negative or both positive, but i do not know how to interpret the electric field graph! I don't understand how the electric field lines tell me anything, they are going neither towards or away from the point charges... I'm trying to think if the asymptotes mean anything but I just don't know where to begin?

 

[cupid.callin]

if both charges were same .. field should have been 0 in between a and b ... which is not .. so i think that they are of opposite sign

 

[danielcheung1]

so are you saying that the electric field graph as shown in the figure is indicative of electric field strength? I still don't understand what the graph is indicating, what those lines stand for?
and if they were of opposite sign, and the graph is symmetrical, how would i go about deciding which charge, a or b, is positive or negative?