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Definition

There are a number of ways to display the data on a chart. A linear chart is just like the charts that you used to draw at school with both the X and the Y axis showing one unit for each interval. A semi-log scale shows one axis (the X-axis) linearly and the other axis (the Y-axis) logarithmically. So the X-axis shows the date at regular intervals. The Y-axis shows the prices logarithmically. The best way to explain this is with an example. Consider the following chart of the JSE Overall index going back to February 1985:
<figure class="container-fluid"><img class="img-responsive" style="display: block; margin: 0 auto; max-width: 100%;" src="/onlinecourse/images/semi-log-1.png" alt="semi-log chart" />
<figcaption style="text-align: center;">JSE Overall Index 1985 to 2017</figcaption>
</figure>
It looks like the 2008 sub-prime crisis was much bigger than the 1987 crash. Now look at exactly the same chart, but in a semi-log form:
<figure class="container-fluid"><img class="img-responsive" style="display: block; margin: 0 auto; max-width: 100%;" src="/onlinecourse/images/semi-log-2.png" alt="semi-log chart" />
<figcaption style="text-align: center;">JSE Overall Index 1985 to 2017 Semi-Log</figcaption>
</figure>
Now you can see that in percentage terms, the 1987 crash was about the same size as the 2008 sub-prime crisis.

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