After a short introduction, I always try to write a paragraph which summarises the main points. When there is a lot of information (like in the bar chart below), it can be difficult to select the main points.
The table below shows the figures for imprisonment in five countries between 1930 and 1980. (The y axis shows numbers of prisoners in thousands)
(Cambridge IELTS 2)
Usually I look for a change from the beginning to the end of the period. However, there is no overall trend because the figures fluctuate. So, I'll talk about the highest and lowest figures instead.
Here is my summary of the main points:
While the figures for imprisonment fluctuated over the period shown, it is clear that the United States had the highest number of prisoners overall. Great Britain, on the other hand, had the lowest number of prisoners for the majority of the period.