Last week I wrote a lesson about summarising the information on a graph. After your summary, you then need to describe specific details. It's important to include numbers and make some comparisons.
Try to write 2 paragraphs describing details. It looks more organised if you divide the information into 2 parts.
For line graphs, I always use the following approach:
1st detail paragraph: compare the numbers for the first year (e.g. 1951), then describe the changes up to a key point on the graph (e.g. peak numbers in 1971).
2nd detail paragraph: explain the general trend for each line after the key point (1971), then compare the numbers for the last year shown (2009).