Line graphs are the most common type of graph used in Science. In the graph above, showing a plant’s height over a period of months, the points are joined dot to dot. This is done when we are looking how something changes over time and not determining the relationship between variables. This kind of graph is very often used in Biology.
In Chemistry and Physics, graphs are normally drawn to establish relationships between variables. In this case the points are NOT joined directly to one another, but a best fit line is drawn.