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Producing Logarithmic Axes

In certain situations you may wish for one or both of the plotted axes to be displayed logarithmically rather than linearly. For instance, you may wish to do this when using a PlotPlot to represent a spectrum of, say, flux against frequency. In this case, you can cause the frequency axis to be drawn logarithmically simply by setting the boolean LogPlot attribute for the frequency axis to a non-zero value. This causes several things to happen:

1. The MappingMapping between the base FrameFrame of the Plot (which represents the underlying graphics world coordinate system) and the base Frame of the FrameSetFrameSet supplied when the Plot was created, is modified. By default, this mapping is linear on both axes, but setting LogPlot non-zero for an axis causes the Mapping to be modified so that it is logarithmic on the specified axis. This is only possible if the displayed section of the axis does not include the value zero (otherwise the attempt to set a new value for LogPlot is ignored,and it retains its default value of zero).

2. The major tick marks drawn as part of the annotated coordinate grid are spaced logarithmically rather than linearly. That is, major axis values are chosen so that there is a constant ratio between adjacent tick mark values. This ratio is constrained to be a power of ten. The minor tick marks are drawn at linearly distributed points between the adjoining major tick values. Thus if a pair of adjacent major tick values are drawn at axis values 10.0 and 100.0, minor ticks will be placed at 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 80.0 and 90.0 (note only 8 minor tick marks are drawn).

3. If possible, numerical axis labels are shown as powers of ten. This depends on the facilities implemented by the graphics wrapper functions (see the next section). Extra functions were introduced to this set of wrapper functions at AST V3.2 which enable super-scripts and sub-scripts to be produced. Some older wrappers may not yet have implemented these functiosn and this will result in axis labels being drawn in usual scientific or decimal notation.

Whilst the LogPlot attribute can be used to control all three of the above facilities, it is possible to control them individually as well. The LogTicks and LogLabel attributes control the behaviour specified in items 2 and 3 above, but the default values for these attributes depend on the setting of the LogPlot attribute. This means that setting LogPlot non-zero will swicth all three facilites on, so long as zero values have not been assigned explicitly to LogTicks or LogLabel.