The string function gprintf("format",x) uses gnuplot's own format specifiers, as do the gnuplot commands `set format`, timestamp, and others. These format specifiers are not the same as those used by the standard C-language routine sprintf(). gprintf() accepts only a single variable to be formatted. Gnuplot also provides an sprintf("format",x1,x2,...) routine if you prefer. For a list of gnuplot's format options, see `format specifiers`.