gnuplot: how to use a*(10^b) format on axis

All format specifiers that gnuplot has to offer can be found here.

If you don't like the 2e+07 format you could try setting the format with

set format x "%.0s*10^{%T}"

which gives you a format like 15*10^6.

Also one can use the following to obtain scientific notation with cross symbol: 2x10^6

set format x "%2.0t{/Symbol \264}10^{%L}"