The Use of Semi-log Graph Paper
Related Topic: How to label semi-log graph paper.
Semi-log graph paper is simply graph paper which has one linear axis and one log axis. It is used in the case where the data range on one axis is extremely large and/or does not follow a linear progression. What this means is that the values increase exponentially. For example, the series 1, 2, 3, 4, 5, 6... is a linear progression, whereas the series 10, 100, 1000, 10000, 100000... is a log progression. If you attempt to create a graph (using the first series vs. the second) on linear graph paper you would get something like the following:
Up front there doesn't appear to be anything wrong with the graph. However, you'll notice that the y-axis data follow a linear progression but the line (had it been drawn) is not straight (as should be produced by these data). Changing the x-axis to a log scale, however, will rectify the problem:
This will make it much easier to extrapolate data because the range is much more spread out.