These are used to compare two sets of data. One set of data is put on the x-axis (the horizontal axis) and the other on the y-axis (the vertical axis). If one set of data depends upon the other, this is put on the y-axis (and is known as the 'dependent variable'). For example, if you were plotting a child's height at various times, the height would depend upon the time and so the height is the dependent variable and goes on the y-axis, whereas time doesn't depend on anything and so is the independent variable and goes on the x-axis. - See more at: http://www.mathsrevision.net/gcse-maths-revision/statistics-handling-data/representing-data#sthash.HYcOfatU.dpuf
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