When a frame is in line, you can move it up or down within the line.
To move an in-line frame up or down
You can drag the frame such that its lower edge is resting on the text baseline, is centered on the line, hangs with the upper edge at the text baseline, or rests anywhere in between. An in-line frame behaves like a word in the text, in the sense that, when you enter additional items to the left of it, the frame is pushed along in the line.
You can also adjust the offset in the Plot Properties + Frame tabbed dialog. After choosing In Line, specify a value for Baseline, Offset from Bottom of Frame. A toggle in the dialog box allows you to choose points, inches, centimeters, or picas as units of measurement.