The chart illustrates the average time to assemble automobiles in America by four different brands (General Motor, Ford, Toyota, Honda) between 1998 and 2005.
Overall, the figures of all brands presented a downward trend in time used to produce vehicles despite some fluctuations. In 1998 (I think specific numbers/years should not appear in overall part), General Motor took the longest time to put together a vehicle, whereas Honda spent the least compared to the rest. You may write " In addition, the time General Motor needed to complete vehicles was the longest among all manufacturers while that of Honda took the shortest generally."
Over 7-year period, the producer General Motor had the greatest improvement on time spent for making cars, which totally decreased 10 hours from 32 hours to 22 hours (no need to mention this b/c you had written "over 7-year period earlier). In 1999 and 2004, Ford and General Motor spent the exact same hours in the making of a vehicle, which were approximately 29 hours and 24 hours respectively.
In 1998, cars completing duration of Honda and Toyota shared a same beginning point, which was about 22hours. Although Honda used three hours less than Toyota did to put together the vehicles in 1999, both manufacturers gradually took the exact hours that rose from 22 hours to 24 hours to produce during 2001 and 2002. Despite some fluctuations between 2002 and 2005, Toyota and Honda eventually ended up spending 20 hours to make a car.