Plotting mantissa against a vector

Given a vector xx=rand(16,1) and to plot its mantissa against xx I simply use

plot(floor(log10(xx)), xx)

This is apparently easy, however it seems not working for vectors containing elements in floating point format, e.g. 4.34e-4 .

Is this the right way to do it or something is wrong? What is good way to represent the the mantissa of these componet of the vector elements against itself?

[darova]

>> floor( log10(4.34e-4) )
ans =
    -4