In the simplest terms it is a horizontal number line which represents real numbers from negative infinity to positive infinity.
 
 
 

2.) The Y-axis: This is the vertical line that on a simple two dimensional graph passes through the origin (the 0,0 position).

In the simplest terms it is a vertical number line which represents real numbers from negative infinity to positive infinity.
 

3. Combining the X-axis and the Y-axis gives you a two dimensional coordinate system, where each data point has two values, the x value and the y value. As an example the origin has the x,y coordinates of 0,0.

Introduction:  While Qualitative observations will suffice for general discussion purposes, sooner or later, if real scientific investigation is to follow, measurements and expression of those measurements in the form of table, charts and graphs must follow.  Being able to transfer data onto a graph and interpret that graph is an essential skill for anyone involved in doing science or learning to do science.

In the process of graphing you will come to quickly recognize that there three types of variables.  These are referred to, as controlled, independent, and dependent variables.

Controlled Variables: Controlled Variables are just that, variables that need to be controlled, that is, held constant.  If they are allowed to change during an experiment they could interfere and affect the outcome in unpredictable ways.  The result of an experiment would be invalid if variables that need to be controlled are not identified and held constant.  We might call a controlled variable by the letter name C or K to represent a value that remains constant.

Independent Variable: The Independent Variable, often call the manipulated variable, is the one variable that the person doing the experiment deliberately changes (manipulates) in order to see what will happen to a third kind of variable referred to as the dependent variable.  On an ordinary graph we would most likely represent this independent variable as the x-variable and we would plot its values on the x-axis. In a two column T-chart of data, the Independent variable is written in the left column.

Dependent Variable: The Dependent Variable is a function of the independent variable, which is the one being deliberately manipulated by the experimenter.  On an ordinary graph we would most likely represent the dependent variable as the y-variable and we would plot its values on the y-axis. In a two column T-chart of data, the Dependent variable is written in the right column.

To understand the terms describing variables, think about the following biology experiment frequently done by students in junior and senior high school.

1. A student sets out to determine the effect of plant food on the growth of plants.

2. In setting up the experiment, the student realizes the growth of the plant will be a function of the amount of plant food used.  Therefore the growth of the plant is the dependent variable because the growth depends upon the amount of food.

3. Likewise, the amount of food is being manipulated by the student and has a direct effect upon the plant growth.  Therefore, the amount of food is the independent variable.

4. Finally, but just as important to the successful outcome of the experiment, the student carefully determines what other factors would include the amount of water used, the amount of light exposure, and the kind of soil the plants were grown in.  All of these variables would need to be kept constant so they would not influence the growth of the plants.  These factors, also called variables, must be controlled.

Special Note: When making charts and graphs there is a structured approach to representing data. 

Rule 1: When you are preparing a chart showing the data for both the independent variable and the dependent you would make a two-column chart (Note: the controlled data need to be shown because it is constant and never changes).  The independent data goes in the first, the left hand, column.  This represents the x-values.  The dependent data goes in the second, the right hand, column.  This represents the y-values.

Rule 2: When graphing data representing the two variables. The independent variable is plotted along the x-axis, while the dependent variable is plotted along the y-axis. Remember that the dependent variable is a function of the independent variable. There is a cause effect relationship here. The independent variable's change causes a change in the dependent variable.