I have thought a way for clearly and simply indicating to which axis each plotted line belongs. I hope no more explanations are needed (if so, I would have not reached my goal :). Different colours could also be used (I did something similar in this paper). 
Plotting in log-scale is often the only way of displaying a given dataset. Unfortunately, log-scales are more difficult to understand even for trained eyes. Often, it is used the formula "note the log-scale of y-axis" in the figure legend. However, it hardly makes the plot easier to follow for log-adverse people. This plot shows an elegant way to display the untransformed data and, at the same time, benefiting from the illustrative power of a log-scale. 
The interesting thing about this plot is the reversed x-axis: from left to right "vegetation height" goes from tall to short vegetation. Note the right-hand arrow indicating this unusual x-axis scale. The authors explain that this was done to "...emphasize that islands with low vegetation are more dissimilar from Staniel than are islands with high vegetation.".
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