Hopefully I explain this well:

I have data logged over a few days. It is simply stored as two columns
of equal length. (Temp and pressure)

I am performing a test on the pressure reading using an IF statement in
the 3rd column that indicates which "mode" the experiment was in at the
time. Say there are 4 types of data modes that are of variable/random
length and spread throughout the data set.

My end result needs to be a graph of each mode (T vs P).

Currently, depending on the mode of data found, I am storing the 1st
mode data in the 4th column, the 2nd mode in the 5th column etc etc...

The problem is I am using IF statements such as:

=IF(C1=1,A1,"")

down the column (example for the 1st mode)

This means that the column has a whole bunch of 'blank' cells with the
"". However, when I go to plot the whole column, these are plotted as
zeros and change the whole graph. If those cells are literally deleted
it works fine.

The whole point of the spreadsheet is that the data just needs to be
pasted into the sheet and will work for any length of data without
having to go through and delete the cells with useless data.

Is there something I should be writing to these cells that will
effectively be seen as a "no value cell" and won't be recognised by the
plotting function?