When you are given two different charts, the overview paragraph is quite easy: just write one sentence about each chart, describing a main feature of each.
Here's the two chart task that we've been looking at in recent lessons:
The charts below show reasons for travel and the main issues for the travelling public in the US in 2009.
And here's my 2-sentence overview paragraph:
It is clear that the principal reason why Americans travelled in 2009 was to commute to and from work. In the same year, the primary concern of Americans, with regard to the trips they made, was the cost of travelling.
I've used several examples of paraphrasing in the paragraph above (when you compare it to the question statement). Can you see them?