Khan Academy
The following link goes to a section of the Khan Academy website devoted to linear graphs. There is a prolific number of videos dealing with various aspects of linear graphs. Also, if you click on the Practice Concept tab in the top right corner of each topic, you have the opportunity to solve practice problems.
https://www.khanacademy.org/math/algebra/linearequationsandinequalitie
Here is the link to an excellent website which has many pages,with applets, relating to linear graphs. This link goes directly to a page explaining gradient. 
http://www.mathwarehouse.com/algebra/linear_equation/slopeofaline.php

Yay Math
The video below from Yay Math shows that very cool dude explaining gradient or slope.
Go via this link to MyMathsOnline and view a lesson about gradient and how to use the Gradient Formula

Go to this link to review how to find the equation of a linear graph from two points on the line. There are some practice questions as well. 
Here is an applet which demonstrates all the different forms that a linear equation can be written in. Drag the red dots to change the position and equation of the line.
! Note: Source code comments as well as public attribution as described http://www.mathwarehouse.com/licensingoverview.php may not be removed or altered>
More Math Applets by www.mathwarehouse.com
Slope Interept Form Point Slope Form Standard Form  Toggle Points Slope? Show Rise and Run ? 
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