two lined x-axis ticklabels

[frankabend]

hello,
i'm working on plots right now which should have to x-tick-labels.
for example: i want to have the x-axis with numeriacal ticks (1 2 3 4 5...)
and in a second line i'd like to have for example a b c d e

it should look like this:

|
|
|
|
|_____________________________
1 2 3 4 5 6 7 8 9
a b c d e

the only way i can think of is by doing it with the text-Function - but the the positions are fixed and if i zoom in the labeling disappears.

do you know a way to do that?

thanks in advance,
alex