Flex charting controls support the following types of axes:

CategoryAxis  

CategoryAxis class maps a set of values (such as stock ticker symbols, state names, or demographic categories) to the axis. You use the <mx:CategoryAxis> tag to define axis labels that are grouped by logical associations and that are not necessarily numeric. For example, the month names used in the chart in About charting could be defined as a CategoryAxis class.

LinearAxis 

A LinearAxis class maps numeric data to the axis. You use the <mx:LinearAxis> child tag of the <mx:horizontalAxis> or <mx:verticalAxis> tags to customize the range of values displayed along the axis, and to set the increment between the axis labels of the tick marks.

LogAxis 

A LogAxis class maps numeric data to the axis logarithmically. You use the <mx:LogAxis> child tag of the <mx:horizontalAxis> or <mx:verticalAxis> tags. Labels on the logarithmic axis are even powers of 10.

DateTimeAxis 

A DateTimeAxis class maps time-based values, such as hours, days, weeks, or years, along a chart axis. You use the <mx:DateTimeAxis> tag to define the axis labels.

The DateTimeAxis, LogAxis, and LinearAxis are all of type NumericAxis, because they are used to represent numeric values. In many cases, you are required to define only one axis as being a NumericAxis or a CategoryAxis. Flex assumes that all axes not explicitly defined are of type LinearAxis. However, to use decorations such as DataTip labels and legends, you might be required to explicitly define both axes.

There are exceptions. For a PlotChart control, both axes are considered a LinearAxis, because the data point is the intersection of two coordinates. So, you are not required to specify either axis, although you can do so to provide additional settings, such as minimum and maximum values. When you create PieChart controls, you also do not specify either axis, because PieChart controls use a single set of data points to draw wedges that represent a percentage of the whole. In PieChart controls, you define a nameField on the chart's data series, rather than a categoryField or name on the axes for labels and legends.

Each axis can have one or more corresponding AxisRenderer objects (specified by the horizontalAxisRenderers or verticalAxisRenderers properties) that define the appearance of axis labels and tick marks. In addition to defining formats, you can use an AxisRenderer class to customize the value of the axis labels. For more information, see Formatting Charts.

The appearance and contents of axis labels are defined by the <mx:horizontalAxis> (x-axis) and <mx:verticalAxis> (y-axis) tags and the renderers for these tags (<mx:AxisRenderer> tags within the <mx:horizontalAxisRenderers> and <mx:verticalAxisRenderers> tags). These tags not only define the data ranges that appear in the chart, but also map the data points to their names and labels. This mapping has a large impact on how the Data Management Service chart renders the values of DataTip labels, axis labels, and tick marks.

By default, Flex uses the chart type and orientation to calculate the labels that appear along the x-axis and y-axis of the chart. The labels of a column chart, for example, have the following default values:

The x-axis  

The minimum number of labels is 0, and the maximum is the number of items in the data series that is being charted.

The y-axis  

The minimum value on the y-axis is small enough for the chart data, and the maximum value is large enough based to accomodate the chart data.

For more information about chart axes, see Working with axes.