How to Graph Data on Semi Log Paper

Semilog paper is graph paper in which one axis is on a logarithmic scale. A logarithm is the inverse of an exponent. Logarithms are the power to which the base has to be raised to equal the number. For example, in base 10, the logarithm of 100 is 2 because 10^2 = 100. Semilog paper is useful when one variable covers a very wide range, or when you are more interested in how a variable changes in terms of ratios than differences. For example, if you were graphing stock prices over time, you might want semilog paper because going from $10 a share to $20 is akin to going from $100 a share to $200 rather than $100 to $120.

Instructions

    • 1

      Gather data as you ordinarily would. For example, on January 1, 1900, the Dow Jones Industrial Average (DJIA) was about 50. On January 1, 1950, it was about 250, and on January 1, 2000, it was about 10,000. (You would plot many more points than this).

    • 2

      Choose endpoints for the axes. The endpoints should encompass the full range of data. In the example, the x-axis should cover 1900-2010, and the y-axis 10 to 15,000.

    • 3

      Choose tick marks. Depending on the size of the graph, 10 to 20 tick marks on each axis should be enough. These should divide the axis equally. In the example, the x axis could be every 5 years, so you would have ticks at 1900, 1905 and so on. On the y-axis, however, the tick marks have to be at multiples of each other. So, you could have tick marks at 50, 100, 200, 400, 800, 1600, 3200, 6400 and 12,800.

    • 4

      Plot the points, using the tick marks as guides.

    • 5

      Add a line connecting the points, if you wish.

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