Oil Price Chart Scale 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.