A straight line graph can contain:
- an x term
- a y term
- a number
So if you only see these terms in an equation, you know you are dealing with a straight line graph. Anything else like xy, a square, a cube, 1/x, 2/y etc won’t be a straight line.
Horizontal and Vertical Lines
- x = a is a vertical line going through a on the x-axis
- y = a is a horizontal line going through a on the y axis
So on the graph above the red line is x = -2 and the blue line is y = 4.
Sloping Lines Through The Origin
Any graph with the equation y = ax or y = -ax is a diagonal straight line though the origin.
So on the graph below the blue line is y = x, the green line is y = 0.5x and the red line is y = -2x.
Plotting Straight Line Graphs
To plot a straight line graph just choose 3 values of x and put these values into the equation to find the y coordinates. Then plot the points. Finally draw a straight line that goes through all three.
To find where it goes through the y-axis set the value of x to be 0 in the equation. To find where it goes through the x axis set the value of y to be zero.
NOTE: You could just plot two points to draw a straight line graph….but it’s best to plot 3 to guard against mistakes.