What interval would be appropriate to graph the data?

This question is incomplete. what you have asked is how much should the numbers go up by on a graph so it all fits on the graph and can be understood.
1st we need to know what data if your numbers were 5 15 20 25 25 20 15 an appropriate interval would be by 5's however if your data was 100 500 1100 800
There would be know way you could fit it on a graph if you went up by five. A better interval would be 100's 

Based on my response I hope you can see that in order to find appropriate interval to graph the data we need the data and that you now see what to do or can update the question

Whose mistake? can you give more details to what the question is asking?