It is stated in the Writing Task 1 Bandscore Descriptor that you should “clearly present a fully developed response” and “present, highlight and illustrate key features clearly and appropriately”. You also have to notice that in the task question, it is said “summarise the information by selecting and reporting the main features”, which means you should have a really good overview or summary of the chart(s).
If there are more than one chart, then you should try to find the connections between them and “make comparisons where relevant”, because the examiner wants to see it.
Here is one academic IELTS Writing Task 1 that my students asked me last Sunday.
In this case, I will try to find the highest and lowest figures in these two tables above, in order to summarise the main features.
The largest figure for consumption is found it Europe.
The largest figure for production is found in Asia, followed by Europe.
The two smallest figures for consumption are found in Africa and South and Central America.
The two smallest figures for production are found in those two regions as well.
Do you see the connections?