page_title: Semi-log plot
section_title: 2009 "swine flu" progression
hierarchical_section_title: Semi-log plot / Real-world examples / 2009 "swine flu" progression
caption_attribution_description: English: Development of H1N1 influenza cases in 2009. Semi-logarithmic version. Source: WHO figures. 中文（简体）‎: 2009年流感确诊案例与疑似案例发展趋势图。对数版本。 中文（繁體）‎: 2009年流感確診案例與疑似案例發展趨勢圖。對數版本。
context_page_description: In science and engineering, a semi-log plot, or semi-log graph, has one axis on a logarithmic scale, the other on a linear scale. It is useful for data with exponential relationships, or where one variable covers a large range of values.
  All equations of the form form straight lines when plotted semi-logarithmically, since taking logs of both sides gives
  This is a line with slope and vertical intercept. The logarithmic scale is usually labeled in base 10; occasionally in base 2:
  A log-linear plot has the logarithmic scale on the y-axis, and a linear scale on the x-axis; a linear-log is the opposite. The naming is output-input, the opposite order from.
  On a semi-log plot the spacing of the scale on the y-axis is proportional to the logarithm of the number, not the number itself. It is equivalent to converting the y values to their log, and plotting the data on linear scales. A log-log plot uses the logarithmic scale for both axes, and hence is not a semi-log plot.
context_section_description: While ten is the most common base, there are times when other bases are more appropriate, as in this example:
caption: A semi-logarithmic plot of cases and deaths in the 2009 outbreak of influenza A (H1N1).  Notice that while the horizontal (time) axis is linear, with the dates evenly spaced, the vertical (cases) axis is logarithmic, with the evenly spaced divisions being labelled with successive powers of two. The semi-log plot makes it easier to see when the infection has stopped spreading at its maximum rate ie. the straight line on this exponential plot, and starts to curve to indicate a slower rate. This might indicate that some form of mitigation action is working eg. social distancing.
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