The primary focus when writing an essay based on a chart should be the overall trend or pattern depicted in the chart, as it provides the main insight into the data being presented. Option B is correct because it emphasizes understanding the big picture. Option A is irrelevant as color does not contribute to the analysis. Option C is incorrect because focusing on specific outliers can overshadow the main trend. Option D is not appropriate because the essay should be based on the chart's data, not personal opinion.