Graph of linear equation in two variable is always a?

A linear equation is an equation between two variables that gives a straight line when plotted on a graph.

Graph of linear equation in two variable is always a straight line because the two variable each have their own unique solutions and point on the graph. 

The graph of liner equation, therefore there always a straight line as two variables are independent of one another, so we get the straight line with unique solution which lead to the straight line.