The Two Kinds of Graph

• Line graphs and bar graphs are visual representations of statistics. Both feature two axes, the horizontal x-axis and the vertical y-axis. The normal convention is for the x-axis to show the known component and the y-axis to show the measured component. Alongside each axis, the items and units being measured are written. Both types need a title explaining what the graph is about, including what units are being used.
  
  

Line Graphs

• Line graphs chart trends.laptop-graph isolated on black image by Olaru Radian-Alexandru from Fotolia.com
  
  Line graphs are used for displaying a single measured variable against a series of known quantities. An example would be charting temperature changes over time. Whenever a thermometer reading is taken, a dot or cross is plotted on the graph where the known time and measured temperature meet, so a reading of 33 degrees after 60 seconds will result in a dot at right angles to 33 degrees on the y-axis and 60 seconds on the x-axis. When all the points are joined up, the line will slope upward or downward to connect the dots (in this example, represent warming or cooling trends).
  
  

Bar Graphs

• Bar graphs compare quantities or volumes.sales plinth green up text image by Nicemonkey from Fotolia.com
  
  Bar graphs compare two or more counted or measured things. The y-axis will usually represent the amount, but along the bottom, under the x-axis, the items being measured will be written beneath each bar. A bar graph could be made to show the numbers of girls and boys in a schoolroom; there would be two bars, one called "Boys", the other "Girls." If there were 15 boys and 12 girls in the class, the "Boys" column would reach 15 on the y-axis, the "Girls" column 12.
  
  

Differences

• The correct type of graph must be chosen.businessman and chart image by Kit Wai Chan from Fotolia.com
  
  Line graphs are theoretically "infinitely precise;" that is, once the points are joined up, any value in between can be worked out. With a bar graph, a measurement is specific to the item counted by its bar.
  
  A line graph measures one property and changes in its value over a consistent base measure (for example, temperature fluctuations over time, which continues more or less consistently). Bar graphs measure and compare multiple things---for example, the numbers of oranges, lemons and limes sold by a company in September.
  
  

A Practical Example

• A seismograph is an example of a line graph. It measures small movements of the earth's crust and larger earthquakes. A pen is constantly drawing a line on paper attached to a slowly rotating drum. Most of the time, the seismograph measures tiny movements, but when there is an earthquake, the pen moves violently and measures a high magnitude.
  
  The simple readout of the seismograph is a line graph.
  
  Using the results of the seismograph, a bar graph could be plotted showing, for example, the number of tremors per month over a year.