This is accomplished via the parameter expression. The user can supply a boolean selection expression to this parameter, involving (for example) names of columns in the source table. The full grammar of such expressions is described in the selectlib library.
An example of a situation in which some selection may be desirable is in the case in which the source list is the standard XMM emldetect pipeline product. These lists contain many rows corresponding to the same source position but with different values of the ID_BAND keyword. Inclusion of all the rows in the table will lead to plots which have many concentric source circles at each position. It is better to use a selection expression such as
expression='(ID_BAND==0)'
to limit the sources plotted to those with ID_BAND = 0 (or whatever other number is desired).
