non-numeric argument to binary operator [closed]

Answer 1

The error is because of the trailing + after geom_point(). Remove that and it should work.

Answer 2

With version 2.0.0, ggplot2 has gained a new geometry, geom_blank() which draws nothing.

It can be used to avoid this type of errors when it is placed as last layer.

plot <- ggplot(data, aes(x, y)) +
  geom_point() +
  #aes(alpha=0.3,color=scale) +
  #scale_color_gradient(high="red") +
  geom_blank()

Using geom_blank() in this way is similar to the workaround in @krlmlr's answer which uses list() as last layer.

Answer 3

Christopher's answer is perfectly correct. Let me add that ggplot also seems to accept lists:

plot <- ggplot(data, aes(x, y)) +
  list(
    geom_point(),
    aes(alpha=0.3,color=scale),
    scale_color_gradient(high="red"),
    NULL
  )

Unfortunately, unlike Python where you can write [1, 2, 3, ], the construct list(1, 2, 3, ) produces an error in R. This is the reason for the final NULL, which is happily ignored by ggplot2.

Another possible workaround is to write

plot <- ggplot(data, aes(x, y)) +
  geom_point() +
  #aes(alpha=0.3,color=scale) +
  #scale_color_gradient(high="red") +
  list()

The final list() is supposed to stay in place to cancel the effects of the last + sign; it is a no-op otherwise.