If a ZAXVAL value is supplied, it is converted into the corresponding ZCOLVAL value. Either way, only vectors for which the Z column value in the catalogue equals the specified ZCOLVAL value are plotted. If the catalogue contains no vectors with the specified ZCOLVAL, then the nearest ZCOLVAL for which some vectors are available is used instead of the supplied ZCOLVAL. The value actually used is reported to the user and included in the default title for the plot.
An alternative method for displaying spectropolarimetry data is to use KAPPA LINPLOT to display line plots of the quantities of interest, as a function of spectral channel. This method has the advantage that, unlike POLPLOT, it can be used even if your data has no spatial coverage (i.e. covers only a single pixel in the x-y plane). Before using LINPLOT you need to create an NDF containing the data to be plotted. To do this, use POLIMAGE. As an example, to plot polarization against wavelength at ( x=1.5, y=2.5 ) for the data in catalogue mycat.FIT, do:
% polimage in=mycat out=pdata coldat=p colvar="dp*dp" box="[5,5,3\]" % linplot ndf="pdata(2,3,)" errbar
POLIMAGE creates the file pdata.sdf which is a 3D NDF containing the values from the ``P'' column in mycat.FIT. The VARIANCE component of the NDF is filled with the square of the values in the ``DP'' column (do not supply a value for parameter COLVAR if the catalogue does not contain a suitable error column). The values assigned to the BOX parameter should usually be the same as the bin sizes used to create the catalogue (use 1,1,1 if the catalogue has not been binned). The LINPLOT command then plots the NDF data values at X and Y pixel indices of 2 and 3 (i.e. pixel coordinates ( 1.5, 2.5 ) ), and covering the entire third (Z) axis. If the catalogue contains data only for a single x-y position, then the LINPLOT parameter NDF could be assigned the value ``pdata'' (without any section specified) since there is no ambiguity about which x-y pixel to use. The ERRBAR parameter indicates that error bars are to be plotted (this would not be possible if we had not assigned a value to the COLVAR parameter when running POLIMAGE).
There are many other options for using these applications. See there
reference documentation for details.