You're right, we do need to see the whole year. The BBC has refused to provide that data numerous times, using the journalism clause as a get-out for FOI purposes.

I don't think my denominator is wrong - it's simply providing a measure of something different. Useful in identifying a basic trend which can then be dug into a little, such as looking at time given as suggested by u/DeathToMonarchs.

If we do look at number of guests it becomes more difficult as we might then want to include unelected commentators and representatives of groups that do not participate in electoral politics, and it skews the comparison with actual electoral support.