If you are graphing data with exponential growth, such as the data describing the growth of a bacterial colony, using the typical Cartesian axes might result in your being unable to easily see trends, such as increases and decreases, on the graph. In these cases, graphing with semi-log axes is helpful. Once you have used Excel to create a set of regular axes, converting the axes to semi-logarithmic axes in Excel is far from difficult.
Type the name of your independent variable at the beginning of the "A" column in Excel. For instance, if you are graphing time versus bacterial growth, type "Time" at the top of the first column. Likewise, type "Bacterial Growth" at the top of the second column (the "B" column).
Use the "Chart" tool to create a graph that plots the independent variable, on the x-axis, versus the dependent variable, on the y-axis.
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Decide which axis you would like to make logarithmic: a logarithmic graph makes both axes logarithmic, while a semi-log graph makes only one of the axes logarithmic.
Double-click that axis. Click on the "Scale" tab, then check the box corresponding to "Logarithmic Scale." Your graph will now become semi-logarithmic.