a bar graph would be best I might say
Any type of graph, really.
A scatter graph.
A line graph would be used for population [change] because you are showing how the population increases or decreases over time. If you are talking about comparing different populations at one given time, then a bar graph or pi chart would be used.
Comparing 2 different item samples.
A pie graph is used for percentage.
a line graph
Bar graphs are used when graphing a qualitative independent variable, but a line graph is used when the independent variable is quantitative. Also when you are comparing quantity of objects(bar graph)
Any kind of graph can be used for quantitative data.
Any kind of graph can be used for discrete data.