This attribute controls the appearance of an annotated coordinate grid (drawn with the AST_GRID routine) by determining the linear interval between the " major" axis values of a Plot, at which (for example) major tick marks are drawn. It takes a separate value for each physical axis of the Plot so that, for instance, the setting " Gap(2)=3.0" specifies the difference between adjacent major values along the second axis. The Gap attribute is only used when the LogTicks attribute indicates that the spacing between major axis values is to be linear. If major axis values are logarithmically spaced then the gap is specified using attribute LogGap.

The Gap value supplied will usually be rounded to the nearest " nice" value, suitable (e.g.) for generating axis labels, before use. To avoid this " nicing" you should set an explicit format for the axis using the Format(axis) or Digits/Digits(axis) attribute. The default behaviour is for the Plot to generate its own Gap value when required, based on the range of axis values to be represented.

Plot

• The Gap value should use the same units as are used internally for storing coordinate values on the corresponding axis. For example, with a celestial coordinate system, the Gap value should be in radians, not hours or degrees.

• If no axis is specified, (e.g. " Gap" instead of " Gap(2)" ), then a " set" or " clear" operation will affect the attribute value of all the Plot axes, while a " get" or " test" operation will use just the Gap(1) value.