How Plot Statements Affect Axis Construction

Primary and Secondary Axes. The LAYOUT OVERLAY container supports two horizontal (X and X2) and two vertical (Y and Y2) axes. The bottom axis (X) and the left axis (Y) are the default axes, referred to as the primary axes. The top axis (X2) and the right axis (Y2) are referred to as the secondary axes and are displayed only if they are requested. For example, consider this simple layout block:

layout overlay;
  scatterplot x=city y=fahrenheit;
endlayout;

Explicitly, the layout block means the following:

layout overlay;
  scatterplot x=city y=fahrenheit / xaxis=x yaxis=y ;
endlayout;

The defaults result in an XY plot having only two axes, X and Y. However, you can request that either the X or Y columns be mapped to the X2 or Y2 axis. The XAXIS= option can be set to X or X2. Similarly, the YAXIS= option can be set to Y or Y2:

layout overlay;
  scatterplot x=city y=fahrenheit / xaxis=x2 yaxis=y2 ;
endlayout;

A single plot statement can activate one horizontal and/or one vertical axis. It cannot activate both horizontal or both vertical axes. Thus, to see both a Y and Y2 axis based on the same Y column, you could specify an additional plot statement:

layout overlay;
  scatterplot x=city y=fahrenheit / xaxis=x yaxis=y ;
  scatterplot x=city y=fahrenheit / xaxis=x yaxis=y2 ;
endlayout;

This layout could be more compactly written as follows:

layout overlay;
  scatterplot x=city y=fahrenheit;
  scatterplot x=city y=fahrenheit / yaxis=y2 ;
endlayout;

Note that this coding produces two overlaid scatter plots, each with five markers. Because the five (X,Y) value pairs and the five (X,Y2) value pairs are identical, the Y and Y2 axes are identical and the markers are exactly superimposed. However, it is not necessary to create a second plot when you want the secondary axis to be a duplicate of the primary axis. A more direct way to accomplish this is shown in Specifying Axis Options.

The next two examples show the independent nature of primary and secondary axes. In each case, a different data column is mapped to the Y and Y2 axes.

layout overlay;
  seriesplot x=date y=price;
  needleplot x=date y=volume / yaxis=y2;
endlayout;

As the following figure shows, the primary and secondary Y axes are independently scaled and there is not a necessary connection between the units or data ranges of either axis.

In the next example, even though the Y and Y2 variables are different, the primary and secondary Y axes represent the same data range in different units. In such cases, the positioning of the tick values on each axis should be coordinated so that the grid lines represent the same temperature on each axis. Apply Axis Thresholds provides example code that shows how to coordinate the tick value positions.

layout overlay;
  scatterplot x=city y=fahrenheit;
  scatterplot x=city y=celsius  / yaxis=y2 ;
endlayout;

Y and Y2 Axes with Coordinated Tick Values