I would suggest that even though the question is written in past tense, the phrase ‘while also’ suggests it changes to present. This is to show the amount the household was spending in that particular moment of time.
If I rewrote the sentence to this:
‘While they were spending $6,000 on food, they were also spending an additional £7,000 on travel.’
Would this make it clearer that the pie chart is showing expenditure at a particular moment in time?
Hope this helps