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It's not possible in ggplot2 because I believe plots with separate y scales (not y-scales that are transformations of each other) are fundamentally flawed. Some problems:

• The are not invertible: given a point on the plot space, you can not uniquely map it back to a point in the data space.

• They are relatively hard to read correctly compared to other options. See A Study on Dual-Scale Data Charts by Petra Isenberg, Anastasia Bezerianos, Pierre Dragicevic, and Jean-Daniel Fekete for details.

• They are easily manipulated to mislead: there is no unique way to specify the relative scales of the axes, leaving them open to manipulation. Two examples from the Junkcharts blog: one, two

• They are arbitrary: why have only 2 scales, not 3, 4 or ten?

You also might want to read Stephen Few's lengthy discussion on the topic Dual-Scaled Axes in Graphs Are They Ever the Best Solution?.

It's not possible in ggplot2 because I believe plots with separate y scales (not y-scales that are transformations of each other) are fundamentally flawed. Some problems:

• They are not invertible: given a point on the plot space, you can not uniquely map it back to a point in the data space.

• They are relatively hard to read correctly compared to other options. See A Study on Dual-Scale Data Charts by Petra Isenberg, Anastasia Bezerianos, Pierre Dragicevic, and Jean-Daniel Fekete for details.

• They are easily manipulated to mislead: there is no unique way to specify the relative scales of the axes, leaving them open to manipulation. Two examples from the Junkcharts blog: one, two

• They are arbitrary: why have only 2 scales, not 3, 4 or ten?

You also might want to read Stephen Few's lengthy discussion on the topic Dual-Scaled Axes in Graphs Are They Ever the Best Solution?.

It's not possible in ggplot2 because I believe plots with separate y scales (not y-scales that are transformations of each other) are fundamentally flawed. Some problems:

• The are not invertible: given a point on the plot space, you can not uniquely map it back to a point in the data space.

• They are relatively hard to read correctly compared to other options. See A Study on Dual-Scale Data Charts by Petra Isenberg, Anastasia Bezerianos, Pierre Dragicevic, and Jean-Daniel Fekete for details.

• They are a easily manipulated to mislead: there is no unique way to specify the relative scales of the axes, leaving them open to manipulation. Two examples from the Junkcharts blog: one, two

• They are a arbitrary: why have only 2 scales, not 3, 4 or ten?

You also might want to read Stephen Few's lengthy discussion on the topic Dual-Scaled Axes in Graphs Are They Ever the Best Solution?.

It's not possible in ggplot2 because I believe plots with separate y scales (not y-scales that are transformations of each other) are fundamentally flawed. Some problems:

• The are not invertible: given a point on the plot space, you can not uniquely map it back to a point in the data space.

• They are relatively hard to read correctly compared to other options. See A Study on Dual-Scale Data Charts by Petra Isenberg, Anastasia Bezerianos, Pierre Dragicevic, and Jean-Daniel Fekete for details.

• They are easily manipulated to mislead: there is no unique way to specify the relative scales of the axes, leaving them open to manipulation. Two examples from the Junkcharts blog: one, two

• They are arbitrary: why have only 2 scales, not 3, 4 or ten?

You also might want to read Stephen Few's lengthy discussion on the topic Dual-Scaled Axes in Graphs Are They Ever the Best Solution?.

It's not possible in ggplot2 because I believe plots with separate y scales (not y-scales that are transformations of each other) are fundamentally flawed.

Some evidence that dual-scale charts perform poorly perceptually can be found in the infovis '11 paper A Study on Dual-Scale Data Charts by Petra Isenberg, Anastasia Bezerianos, Pierre Dragicevic, and Jean-Daniel Fekete.

It's not possible in ggplot2 because I believe plots with separate y scales (not y-scales that are transformations of each other) are fundamentally flawed. Some problems:

• The are not invertible: given a point on the plot space, you can not uniquely map it back to a point in the data space.

• They are relatively hard to read correctly compared to other options. See A Study on Dual-Scale Data Charts by Petra Isenberg, Anastasia Bezerianos, Pierre Dragicevic, and Jean-Daniel Fekete for details.

• They are a easily manipulated to mislead: there is no unique way to specify the relative scales of the axes, leaving them open to manipulation. Two examples from the Junkcharts blog: one, two

• They are a arbitrary: why have only 2 scales, not 3, 4 or ten?

You also might want to read Stephen Few's lengthy discussion on the topic Dual-Scaled Axes in Graphs Are They Ever the Best Solution?.