Does the graph of every line represent a direct variation equation

Depending on where the lines goes through. If the line doesn't go through the origin, then it's not direct variation. If the line does go through the point (0,0), then it's direct variation. You will know if the equation is in y=mx + b form, for the equation. Not always will that form give you direct variation, only when you have an equation that is Y=mx. For example this equation y=3x, is direct variation because, you can multiply the equation by zero. 

Hope this helps!!!