If you read any of my task 1 answers, you will see that I always write 4 paragraphs, and I use a "general to specific" essay structure.
- The introduction is the most general part of the report; it tells the reader what the chart is about.
- Then I write a short paragraph about the main / general points.
- Finally, I write 2 paragraphs describing specific facts or figures.
- I don't write a conclusion because I have already summarised the information in paragraph 2.
One reason why I prefer to put the overview / summary near the beginning (rather than at the end) is because I think it's easier to describe general things first, then specific things later.