You can show a static footer for each column, which is always visible on scrolling. In order to show a footer, just set the footerrow to true.

Each column has a property called Footer Value. It is a string property and you can set it to any value you need
using the callGridMethod with footerData (see the example)
If you need to use formula (like sum, count, average, min, max, etc) - you can use the available property
summarry rows. This is array, which is passed to the renderer. The first argument is the name
corresponding to the name in colModel, the second argument is array which describes to
which field from query should be applied the formula. In this case we set SUM.
It can be any database valid function which accept single argument

For detailed implementation, please take a look at the PHP source code tab.