For question 4.4 in June 2017, the values follow a curve when plotted. This is because energy is required to convert the substances burnt from their standard states to gaseous states. Since the points follow a curve, a curve of best fit is drawn. A line can be straight or curved.
For question 8(b)(i) in the second paper, the values clearly follow a line. This is to be expected since the volume is directly proportional to the mass. So a straight line is drawn.
You have to look at the trend which the points follow and draw a line which is appropriate. If the relation is linear, a straight line is drawn. If not, a smooth curve is drawn.
I made a scatter plot for question 4.4:
https://imgur.com/a/4FfK2WpA straight line is clearly not appropriate
A smooth curve needs to be drawn.