Quite a few of the visual definitions allow the formatting of the numerical values to be plotted. E.g. in MSYMB there is the symbol_format parameter. The formatting has to be defined by using the FORTRAN formatting syntax and has to put into parentheses. E.g.
mv.msymb(symbol_format="(F3.1)")
See the Coloured Observation Values Example from the Gallery for fully-fledged example.