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A detailed step by step tutorial is now available: Use case: mapping sparse spatial data with TOPCAT

1) Standard way (density map from a lon/lat table):

• Open the SkyPlot panel, select table and Lon & Lat parameters in Position panel


• In the Form panel, you can save the healpix map (cell #, plotting parameter) - saved at scale 12, apparently (FITS formatting is more efficient than VOTable, as it is compressed).
You can also load an existing map from there.

2) Alternatively (from a binned table):

This other method is adapted when you have only 1 or 0 data / healpix cell, ie if the grouping has already been done in a previous step (e.g., with an ADQL query to group data by healpix cells). It is  not intended to group or average table rows in the cells.