Help me to improve my English writting skill. Thank so much in advance

I am going to have the writing test 2 weeks later. Since I am not confident at my writing skill at all, I come here to find some helps. I will try to post my writing works everyday, and hope teachers and other students here will check and fix my grammar and writing style:up:. Thank you so much in advance. My first writing is below
Question: the chart below shows the cost of buying two computers in three different shops since the year 2000. Using the information from the chart, write a short report comparing the prices for the computers
( cause the site required me to have as much as 5 posts in order to have a posted link here. Therefore, I paste the link directly here, just copy and paste then u will c the chart, thx :P
i195.photobucket.com/albums/z211/kidofgod_001/DSC00098.jpg

my essay:
First of all, as we see in the chart, the price of the computers was gradually going down as time passing by. In 2000, the highest price for Cotel computer was nearly as much as 2500 USD. In the 2005, the Cotel computer's price significantly decreased by 1600 USD. The Mini-box computer had the same story. It cost not cheaper than 2200 USD in 2000, and drop to less than 1700 USD in 2005. This phenomenon is the result in increasingly using laptop instead of computer of the customer. Secondly, according to the chart, the prices of two kinds of the computers between the three companies were not much different from 2000-2005. Ubit X asked for a little bit higher price of Cotel computer compared with the other companies in the first two years (2000 and 2001). However, in the next three year, the Our-time was the company who had the highest price for Cotel. About the Mini-box computer, WBH usually charged it the most to the customer in the period 2000-2005.