I am drawing a bar chart of age of firefighters.
In the X axis I have the age interval and in the y axis I have numbe of
firefighters with age in that range.

The table is as follows:
Age    No
20    0
30    4
40    64

In this table, 20 means that those whose age are between 0 and 20 and 30
means ages between 20 and 30. Chart draws the bar exactly above these
numbers, but for better interpretation I want the bar chart to appear between
these number. I think this is a basic question and there is a solution for it.
One solution is to put the age as follows:
0-20
20-30
30-40
in x axis but this is not a good solution.