
 Here is how to overlay analysis and forecast fields
 in order to compare them:

 Data icon 'T_an+T_fc48' contains three analysis fields and
 three forecast fields that are valid at the same time. With
 the aid of 'an' and 'fc48' icons we can filter out subsets
 of those fields:

 - GRIB filter icon 'an' picks only analysis fields; test it
    by visualising 'an' icon and scrolling through the fields

 - GRIB filter icon 'fc48' picks only forecast fields; test it
    by visualising 'fc48' icon and scrolling through the fields


 Now let us plot both sets of fields in the same display
 window:

 - visualise the icon 'Europe lat-lon' to create a display
    window covering the European area

 - drop the icon 'an' into your display window - analysis fields
    will be plotted with the default blue isolines

 - select the two icons 'fc48' and 'red-dotted' to drag them
    simultaneously into your display window - forecast
    fields will be plotted with red isolines and overlaid
    on top of analysis fields
