Extracts from: http://www.ieltshelpnow.com/academic_writing_test_1.html
For the IELTS Academic Task 1 Writing, you should have a paragraph for your small introduction, a paragraph for each graph that you are describing and a paragraph for your ending. If there's only one graph to be described, then you should split your writing into 2 or maybe 3 paragraphs for the one graph.
You don't need much here. You only have 150 words to fully answer the question and this is not much. So, you need 1 or 2 sentences describing the following:
The type(s) of graph you are describing
The titles of the graph(s)
The date of the graph(s)
*What I mean by the scale is whether the graphs are marked in hundreds, thousands, millions, pounds, dollars (US, Canadian, Australian, New Zealand, etc.), kilograms, tons, metres, kilometres, percent and so on. It's important for you to make clear what your numbers mean for an accurate report of the graph.
Describing graphs: Remember the function of many graphs is to describe a trend so be sure that you describe the trends. A trend is how values change generally over time and it is important to describe the changes along with some of the individual values.
If there is only 1 graph and it doesn't have much numerical data in it, then you will be expected to include all or nearly all of the numerical detail. If, however, you have 2 graphs, both of which are very complicated with lots of values, you will not be expected to include everything.
Describing bar charts: Basically, with a bar chart, you need to describe the bars and their values. When describing a bar chart you first have to decide in what order to describe the bars, highest value to lowest value or lowest value to highest value. It may be a mixture of this. If there are very many bars, you can sometimes group together for description 1 or 2 or 3 bars which have similar or the same values. If there are very many and you can't group them, then just describe the ones that are the most significant.
The same advice is useful when describing tables and pie charts.
Describing line graphs: The function of a line graph is to describe a TREND pictorially. You therefore should try and describe the trend in it. If there are many lines in the graph(s), then just generally describe the trend. If there is only one or two, then use more detail. So, describe the movement of the line(s) of the graph giving numerical detail at the important points of the line.
Find suitable expressions and words to be used in this case in the 'line graph' section of the webpage cited.