Right-click on the x-axis, choose "Format Axis." In "Scale," check the box
"Logarithmic Scale," then OK. You will have a graph with a logarithmic
x-axis and linear y-axis (semi-log).
Incidentally, Excel will point out to you (over and over again!) if any
logarthimically-plotted values are zero or negative, or if the minimum of the
axis scale is set to zero or below.
