rggplot2scalecategorical-datar-factor

Plotting with ggplot2: "Error: Discrete value supplied to continuous scale" on categorical y-axis

The plotting code below gives Error: Discrete value supplied to continuous scale

What's wrong with this code? It works fine until I try to change the scale so the error is there... I tried to figure out solutions from similar problem but couldn't.

meltDF <- data.frame(
  MW = c(
    3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9,
    9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4,
    8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4,
    7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9,
    6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4,
    3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9,
    9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4, 8.1, 9, 9.4, 3.9, 6.4, 7.4,
    8.1, 9, 9.4
  ),
  variable = factor(
    c(
      "10", "10", "10", "10", "10", "10", "33.95", "33.95", "33.95",
      "33.95", "33.95", "33.95", "58.66", "58.66", "58.66", "58.66",
      "58.66", "58.66", "84.42", "84.42", "84.42", "84.42", "84.42",
      "84.42", "110.21", "110.21", "110.21", "110.21", "110.21", "110.21",
      "134.16", "134.16", "134.16", "134.16", "134.16", "134.16", "164.69",
      "164.69", "164.69", "164.69", "164.69", "164.69", "199.1", "199.1",
      "199.1", "199.1", "199.1", "199.1", "234.35", "234.35", "234.35",
      "234.35", "234.35", "234.35", "257.19", "257.19", "257.19", "257.19",
      "257.19", "257.19", "361.84", "361.84", "361.84", "361.84", "361.84",
      "361.84", "432.74", "432.74", "432.74", "432.74", "432.74", "432.74",
      "506.34", "506.34", "506.34", "506.34", "506.34", "506.34", "581.46",
      "581.46", "581.46", "581.46", "581.46", "581.46", "651.71", "651.71",
      "651.71", "651.71", "651.71", "651.71", "732.59", "732.59", "732.59",
      "732.59", "732.59", "732.59", "817.56", "817.56", "817.56", "817.56",
      "817.56", "817.56", "896.24", "896.24", "896.24", "896.24", "896.24",
      "896.24", "971.77", "971.77", "971.77", "971.77", "971.77", "971.77",
      "1038.91", "1038.91", "1038.91", "1038.91", "1038.91", "1038.91"
    ),
    levels = c(
      "10", "33.95", "58.66", "84.42", "110.21", "134.16", "164.69",
      "199.1", "234.35", "257.19", "361.84", "432.74", "506.34", "581.46",
      "651.71", "732.59", "817.56", "896.24", "971.77", "1038.91"
    )
  ),
  value = c(
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0
  )
)

The plotting code:

## Plotting
ggplot(meltDF[meltDF$value == 1,]) + geom_point(aes(x = MW, y = variable)) +
  scale_x_continuous(limits=c(0, 1200), breaks=c(0, 400, 800, 1200)) +
  scale_y_continuous(limits=c(0, 1200), breaks=c(0, 400, 800, 1200))

Here's how the plot looked before adding scale:

Plot

Solution

  • As mentioned in the comments, there cannot be a continuous scale on variable of the factor type. You could change the factor to numeric as follows, just after you define the meltDF variable.

    meltDF$variable=as.numeric(levels(meltDF$variable))[meltDF$variable]

    Then, execute the ggplot command

      ggplot(meltDF[meltDF$value == 1,]) + geom_point(aes(x = MW, y =   variable)) +
     scale_x_continuous(limits=c(0, 1200), breaks=c(0, 400, 800, 1200)) +
     scale_y_continuous(limits=c(0, 1200), breaks=c(0, 400, 800, 1200))

    And you will have your chart.

    Hope this helps