Suppose you have a histogram like this
3 x
4 xxxxxx
5 xxx
9 xxxxx
The total number of samples is 1+6+3+5 = 15. The middle one of those is sample number 8. Its value is 5.
Suppose we add a sample with a value 3 to the list. Now the middle two samples are the 8th and 9th ones. The 8th sample has a value of 4, and the 9th sample has a value of 5, so the median is (4+5)/2 = 4.5
3 x
4 xxxxxx
5 xxx
9 xxxxx
The total number of samples is 1+6+3+5 = 15. The middle one of those is sample number 8. Its value is 5.
Suppose we add a sample with a value 3 to the list. Now the middle two samples are the 8th and 9th ones. The 8th sample has a value of 4, and the 9th sample has a value of 5, so the median is (4+5)/2 = 4.5