A logarithmic scale by definition goes up in decades. You have to use
multiples of 10, by this I mean 0.01, 0.1, 1, 10, 100, 1000 etc. You
cannot use anything else
It's true that Excel's chart engine is limited in this way, but I have
seen other graphing programs that do allow you to set logarithmic
scales to other than integer powers of 10. Are you in a position to
use logarithms on a regular scale (instead of actual values on a
logarithmic scale) for your X values?