I found the axis on the following graph of crude oil prices over 1950-2015 from MacroTrends surprising. The y-axis labels are 20, 40, 60, 80, 100, ...but they are not equidistant. Why?
Usually, when people use non-equidistant axis, it is because they want to emphasize some variation more than others. For example, if most of your data is between 0 and 1, but you have one outlier on 100, then an equidistant axis would make it hard to analyze most the variation, because it emphasizes the wrong parts.
In this case you have a log-scale, which is a simple nonlinear axis that (in general) emphasizes small values over larger ones. Whether that makes sense with oil prices, is - to me - unclear.
As it has already been explained, it's a logarithmic scale (also called log scale for short). The distance between values doesn't depend on their linear difference, but on their relative difference. The distance between 20 and 40 is the same as the distance between 40 and 80 or the distance between 60 and 120, because 40/20 = 80/40 = 120/60 = 2 or a 100% increase, a fixed rate.
An additional caveat about log scales is that they never go down to 0. After all, if you increase 0 by any percentage, you still get 0. The distance between 0 and any positive value is infinite.