After describing the main features or general trends shown on a graph or chart, we need to describe specific details. It's important to include some numbers and make some comparisons.
Whenever a chart shows years, I describe the details starting with the first year and the highest figure(s). Here's an example paragraph about the year 2006:
In 2006, 100% of mobile phone owners used their phones to make calls, while the next most popular uses of mobiles were for text messaging (73%) and taking photos (66%). By contrast, less than a fifth of owners played games or music on their phones, and there were no figures for users doing Internet searches or recording video.
After this, I would write a final paragraph containing a few key numbers for the other two years (2008 and 2010). I'll show you my full essay next week.