What produces a graph in a straight line

Linear equations in one variables shows always graph in straight line,

For ex: x = 3, y = 4, x = - 1.5, y = - 2/3

Also two variables linear equations with degree of 0 or 1 (Each term) shows always graph in straight line,

For ex: x + y = 5, 2x - 4y = 7 etc.