Charting non-linear data samples on 3d surface plot

Hi all,

I'm trying to plot some experimental data (light intensity at different
positions and different voltages) on a 3d surface plot. However, the
voltages (series) weren't taken at constant intervals. Sample data is:

Voltage
0 1 2 2.2 2.4
-----------------------------------------------
1mm 0 0 0 0 0
2mm 0 1 2 2 3
3mm 0 2 2 2 3
4mm 0 2 3 3 4
5mm 0 3 4 4 5
6mm 0 3 4 5 5

When I try to plot this, the voltage axis plots all data at constant
intervals (presumably corresponding to the columns), regardless of the
value heading at the top of the column, whereas the gap between 1V and
2V should be 5 times larger than the gap between 2V and 2.2V.

Can anyone please tell me how to get Excel to plot this correctly?

Many thanks,
Adrian