Wong, considering that writing task 1 always involves description, comparing and contrasting the information presented in a line graph or a chart, I want to give you some tips that would help you to meet the minimum 150 word criteria in a jiffy, while maintaining accurate analysis of the content of chart or graph. Firstly, having paraphrased the prompt in your opening statement, observe the graph quickly and take note of trends that are immediately obvious on the chart. Those trends will form the basis of your overview statement. For instance, considering the graph in your post, here is an overview:  
  
'Overall, there was a general decline in the level of unwanted materials generated in company A while the company C counterpart recorded a rise. Also, the increase in the amount of wastes produced in company B was interrupted by a sharp progressive decrease. Thus, the two intercepts formed as the slopes of companies A and B cut across that of company C show some level of similarities in the quantity of wastes produced by these manufacturing firms.'  
  
I just gave an overview of the obvious trends in your graph and I have 76 words already. Yours may not really be like mine as you practise, but keeping in mind the way I tried to capture the information in three sentences would help you to come up with a better overview in your next practice tests. This overview should be a paragraph as you rightly created in your current draft. In the next paragraph, you are meant to look critically at each of the slopes for the various companies and describe the decline, the rise, the rise and fall, and the similarities you mentioned in your overview. This time, you should relate your statement, where necessary, to the actual value of the wastes and the corresponding periods of production. See the next example to that effect:  
  
'On the one hand, company A experienced a sustained decrease in the amount of wastes from 12 to 8 tonnes between 2000 and 2015. There was a sharp increase from 8 to 11 tonnes of wastes generated in company B within a five year period from 2000 to 2005, although the latter experienced a progressive decrease from 11 to 3 tonnes in the next decade that followed. On the other hand, the production of unwanted materials rose from 4 to 6 tonnes in company C from 2000 to 2005, remained fairly increased from 6 to 7 tonnes from 2005 to 2010, and then experienced another sharp increase from 7 to 10 tonnes in next five years after 2010.'  
  
Notice that I have not talked about the similarities at the intercepts. So, by the time you discuss that, you shall have written more than 150 words. Remember, the instances above are meant to let you have a feel of how to approach the writing task 1 during your practice tests. You should time yourself during practice so as to check your writing speed and vocabulary development. I do not know how imminent your real test is, but I do hope that these tips would help you to improve faster. Take note that as soon as your are done with the paragraph depicting the second instance as shown above, having written 3-4 sentences, you must conclude your essay and then do a review. I hope these suggestions will improve your response to this aspect of IELTS writing test.