Chart of time dependent variable

Hi

In an experiment, I am changing the voltage every few minutes. I would like to make a plot of how this controlled variable changes every few minutes, with another series on the same chart, showing how the output variable, pressure, changes.
Attached is a excel spreadsheet showing some example data. There is a plot of pressure against reading number [i would like this to be time against pressure but it doesn't work!?]. On this same plot i would like voltage series as well. On the data sheet is how i change the voltage with time. How can i plot this on the chart other than putting a voltage value next to every reading??
Thanks for any help, suggestions etc

Nasal