You can
put this solution on YOUR website!year number of cats Change
1998 30
1999 35 +5
2000 50 +15
2001 35 -15
2002 40 +5
2003 55 +15
Using your data the increase in stray cats from 1999 to 2000 was 15 cats and also between 2002 and 2003 the increase was 15 cats.
On looking at your graph it should be the steepest for these two sets of two years.