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#### A.4.3.5 Aux Axis Control

The Aux Axis control () is only visible when at least one layer is using the shared colour map. The following plot layers do this:

- Aux colour mode
- Weighted colour mode
- Grid form
- Sky Density form
- Healpix control
- Line form
- Line3d form
- Spectrogram control

These all colour parts of the plot in a way that is quantitatively significant, and the Aux Axis control gives you a chance to control the details of the value-colour mapping, and to display the mapping in a colour ramp displayed beside the plot. Some other layer types (e.g. Density mode) also shade the plot according to numeric values, but use their own colour map, and don't display the colour ramp.

If no layer is using the shared colour map, then this control will not appear in the stack.

When present, this control has three tabs, Map, Ramp and Range, described below.

Map Tab

Aux axis control Map tab

The Map tab controls the aux axis colour map. It has the following options:

Aux Shader
  Select the colour map from a list of options. The available colour maps are listed in Appendix A.4.7.
Shader Clip
  Select a sub-range of the full colour map above. If the Default checkbox is checked, then all or most of the colour ramp from the Shader control is used. If you want to configure the range of colours from the map yourself, uncheck the Default checkbox, and slide the handles in from the end of the slider to choose exactly the range you want.

  The default range is clipped at one end for colour maps that fade to white, so that all the plotted colours will be distinguishable against a white background. That is generally a good idea for scatter-type plots, but may not be so good for plots where the whole plotting surface is coloured in, like density maps or spectrograms, so in that case you might want to uncheck Default and leave the handles at the extreme ends of the slider.

Shader Flip
  Whether the aux scale should map forwards or backwards into the colour map.
Shader Quantise
  Allows the colour map to be quantised. By default, the colour map is effectively continuous. If you slide the slider to the right, or enter a value in the text field, the map will be split into a decreasing number of discrete colours. This can be used to generate a contour-like effect, and may make it easier to trace the boundaries of regions of interest by eye.
Scaling
  Determines the function used to map the range of aux data values onto the colour map. The options are:
  - `linear`
  - `log`
  - `histogram`
  - `histolog`
  - `asinh`
  - `square`
  - `sqrt`
  - `acos`
  - `cos`
  In all these cases, the full range of data values is displayed on the colour bar (though it can be restricted by using the Aux Subrange control in the Range tab, described below). The `linear`, `log`, `asinh`, `square`, `sqrt`, `acos` and `cos` options just apply the named function to the full data range, in some cases with some normalisation. Note that `log` scaling will ignore any negative values. The histogram options on the other hand use a scaling function that corresponds to the actual distribution of the data, so that there are about the same number of points (or pixels, or whatever is being scaled) of each colour. The histogram options are somewhat more expensive, but can be a good choice if you are exploring data whose distribution is unknown or not well-behaved over its min-max range. The `histogram` and `histolog` options both assign the colours in the same way, but they display the colour ramp with linear or logarithmic annotation respectively; the `histolog` option also ignores non-positive values.
Null Colour
  What colour should be used to represent points with a null value for the aux data coordinate. If the associated Hide option is selected, then those points will not appear in the plot at all.

Ramp Tab

Aux axis control Ramp tab

The Ramp tab controls the display and annotation of the colour ramp that displays the colour map on the plot. It has the following options:

Show Scale
  Whether the aux scale ramp is visible; if so an appropriately labelled colour ramp appears at the right of the plot. The associated Auto option makes this decision automatically: if any aux data is plotted, the scale will appear, otherwise it won't. Deselect Auto if you want to determine visibility by hand.
Aux Axis Label
  Selects the axis label to be displayed near the aux colour ramp if it is visible. The associated Auto option, if selected, uses the name of one of the coordinates supplying aux data; deselect Auto if you want to enter a label by hand.
Aux Tick Crowding
  The slider influences how many tick marks are drawn on the colour ramp.

Range Tab

Aux axis control Range tab

The Range tab lets you enter lower and upper values for the aux data range by hand, and provides a double slider to restrict the range within these limits. If either the lower or upper range is left blank, it will be determined from the data.

The Lock Aux Range checkbox controls whether the aux range is automatically updated as the plot is adjusted (for instance as you navigate around it with the mouse). If locked, the range will stay the same, but otherwise (the default) it is dynamically updated for the current view of the plot. This checkbox is a duplicate of the toggle button on the plot window toolbar described in Appendix A.4.2.

Note the font used for labelling the aux axis is currently controlled by the font from the Axes selector.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk