In Writing task , The Introductory paragraph is generally written in Simple present tense. While developing data explanation, stick to the timeline i.e. if its in the context of past, use simple past, in context of present,use simple present tense. Theres a possibility of using continous tense along with the simple present tense in a sentence where two clauses are used. Similarly if graph is given in Future tense or projections then use simple Future tense.