In academic writing, we often need to describe a trend of data for example in a line graph. And this is also a kind of writing required in IELTS.
Usually, we can use verbs to depict changes and adverbs to show the degree of changes. Taking the following graph (the price of rice)for example.
We can say 'the price fluctuated wildly' for a, 'the price declined slightly' for b, and ' the price increased gradually' for c.
I conclude the common-used word as follows:
verb: upward trend: increase, rise, soar, shoot up, climb, go up
downward trend : fall, decrease, decline, plunge, drop, down over
others: level off /at, fluctuate, experience(v)
adverb: slow: slowly, slightly, mildly, gradually, marginally, steadily
fast: dramatically, drastically, wildly, sharply, suddenly, remarkably, significantly, noticeably