A line graph shows the value of one variable and how it changes against the value of another. It gives a picture of the trend, and in particular what the latest situation is when plotted against any earlier one.
Common example are sales figures (number sold in a certain time span) and costs according to units produced.
A graph has a horizontal (x) axis and a vertical (y) axis, each one with a particular scale. The scale does not have to start at zero, so long as this is made clear. Additionally, the scale should be appropriate in order to present a clear picture – and this applies particularly to OHTs, of course.
Additional lines can be added, for example to compare more than one product, but again, this should not be too cluttered. The visual picture is supposed to simplify the written or spoken word, not to complicate it.