In Bar graph,
We made a table to a bar graph
Class 
5 ^{ th } 
6 ^{ th } 
7 ^{ th } 
8 ^{ th } 
9 ^{ th } 
Number of girls  30  35  20  25  40 
But,
What if we have data like this
Class 
5 ^{ th } 
6 ^{ th } 
7 ^{ th } 
8 ^{ th } 
9 ^{ th } 
Number of girls  30  35  20  25  40 
Number of boys  20  40  25  35  20 
Here, we make a double bar graph.
Since number of students are from 20 to 40
We draw bar graph from 0 to 45
with a gap of 5
Let’s take another example
Suppose population of India & China are

1951 
1961 
1971 
1981 
1991 
2001 
Population of India (in crores) 
36 
44 
54 
68 
84 
102 
Population of China (in crores) 
58 
64 
70 
90 
100 
110 
Here, population is from 36 to 110
we draw graph from 0 to 110 with gap of 10