Quite often a chart is drawn using logarithmic scale for one axis (usually the the y-axis). This is often used for abuse when presenting information - logarithmic scale alters greatly how the values are plotted on the plane.
Still I guess there are legitimate cases when using logarithmic scale for one axis which represents a value which is linear in nature (not earthquake magnitude which is already logarithmic itself).
What are examples of such legitimate cases where using logarithmic scale allows for better analysis of plotted data?