Dynamic Updates for Randomly Dispersed Scatter Plot Entries (I want a CHARTIF function!)

Hello,

I am putting together a spreadsheet which will have an ever growing number of data entries that will all be catagorized on one of a finite number of data sets. I would like to do the equivalent of a CHARTIF function in order to select the data for the scatter plot, if such a function existed.

For example, in the simple spreadsheet attached, in order to select the X-Axis data entries for the Ford data series, I would type the following into the 'select data' field:
=CHARTIF(A2:A1000,E3,C2:C1000)


Thanks for any help in the right direction!
Grant