Using this window you can change the display of multiple spectra so that an offset in data value is artificially applied so that the spectra can be viewed stacked one above the other. It can also be useful when say comparing spectra from different times or physical state, as the stacking order can be defined as an algrebraic expression that can include time and dates or the actual offset as an expression of some other physical parameter, like period.
In both modes the offset is a value in the data units of the spectra, which must all be the same, no flux matching is supported.
If supplying an order expression it should evaluate to a number for each spectrum, higher values having their associated spectra placed above lower valued ones by the given offset.
If supplying a offset expression it should evaluate to a physical value, so scaling to appropriate values is often necessary.
Note that the values of any properties associated with the spectra are available for use as part of the expression, usually these are the values of the FITS headers and the keywords should be used.
The expressions are the same as used in the TOPCAT application (see SUN/253 or the TOPCAT builtin help), so include all the special functions it provides, for instance isoToMjd will convert an ISO 8601 date into an MJD, which is suitable as a order value. As an example the expression isoToMjd(DATE_OBS) will convert the DATE-OBS value into an MJD, provided the DATE-OBS value is in ISO 8601 format. Note that the minus sign in DATE-OBS has been changed into an underscore, that is deliberate. Any character in a keyword that cannot be a Java identifier will be converted into an underscore (allowed characters are any alpha-numeric, plus the underscore).