Vital. Without a clear summary of the main trends (the "big picture"), you cannot score above a Band 5 or 6. Avoid using specific numbers here; save those for the body paragraphs. 

No. You must "select and report" only the main features. Writing about every tiny change makes your report messy and mechanical. Group similar data together to show you can analyze patterns. 

Yes. Confusing "thousands" with "millions" or raw numbers with percentages is a major accuracy error. Always double-check the legend and the axes before you start writing. 

Never. Do not include your own opinions or outside knowledge (like "sales fell due to the pandemic"). Stick strictly to describing the information shown on the page. 

Vary your language. Don't just say "increased" or "decreased" five times. Use a mix of verbs (soared, plummeted, stabilized) and nouns (a sharp rise, a slight dip) to show the examiner your range. 

To hit the C1 level, try using adverb-verb and adjective-noun combinations to describe the same data:

• Adverb-Verb: "Prices rose significantly."

• Adjective-Noun: "There was a significant rise in prices."