I want to display a set of two different temperatures in a graph ...
It looks like you are using an XY Graph when what you probably want is a Waveform Graph.
As it is, with your XY graph you're plotting one selected temperature vs. the other selected temperature.
Take a look at the context help or the Chart/Graph examples that ship with LabVIEW for more info on the different attributes of these things.
Addendum to my own completely wrong post:
Ian is of course correct and I really need to get new glasses.
To do this you will have to keep a history record of all 10 temperatures yourself and when the radio buttons change extract the two new temperatures from this store and place them into the chart history. You need to set up a 2D circular buffer (there are examples of this if you do a search) but a simple way of coding this is to use a second chart and turn off the visibility. The sample code (LV7.1) gives you some ideas. I had to change from a 'cluster' to a 'build arry' when collecting data for the chart otherwise the method doesn't work.
I hope this helps