In last Thursday's lesson I showed you a graph that would never appear in an IELTS test. The reason why you won't see that type of graph is that it only has one line and there's nothing to compare it with.
IELTS line graphs always have two or more lines, and your job is to compare the lines; don't describe each one separately.
This also means that it is pointless to practise sentences like this:
"The percentage of x rose gradually between 2005 and 2008, and then remained stable for the following two years, before dropping again in 2011 and reaching its lowest point in 2012."
It's unlikely that you'll ever write such a long sentence about only one line on a graph. Hopefully you'll be too busy comparing figures for more than one line!