Something that occurs to me: there's [not much|nothing] special about "dual" axis charts, meaning: in general the graph matches color with the axis, but there's

Something that occurs to me: there's [not much|nothing] special about "dual" axis charts, meaning: in general the graph matches color with the axis, but there's nothing "right" vs. "left" about the graphs. Therefore, there's nothing special about "dual" axis charts being right- and left-axis, and you could just as easily put both axes on the left (at a little loss in clarity). And finally, that means you could just as easily have three, four, or more axes. Not really -- obviously a chart with six axes on the left and six different color-coordinated graphs would be absurd, but that's my point: there's nothing that makes a two-axis graph less absurd other than scale. All of which to say: if there's no real correlation that you're trying to illustrate between the data series, then separate charts are the way to go. If there is a correlation, there's (probably) a better way to illustrate it.