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

Question

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.

This is a head of my data:

> dput(head(df))
structure(list(`10` = c(0, 0, 0, 0, 0, 0), `33.95` = c(0, 0, 
0, 0, 0, 0), `58.66` = c(0, 0, 0, 0, 0, 0), `84.42` = c(0, 0, 
0, 0, 0, 0), `110.21` = c(0, 0, 0, 0, 0, 0), `134.16` = c(0, 
0, 0, 0, 0, 0), `164.69` = c(0, 0, 0, 0, 0, 0), `199.1` = c(0, 
0, 0, 0, 0, 0), `234.35` = c(0, 0, 0, 0, 0, 0), `257.19` = c(0, 
0, 0, 0, 0, 0), `361.84` = c(0, 0, 0, 0, 0, 0), `432.74` = c(0, 
0, 0, 0, 0, 0), `506.34` = c(1, 0, 0, 0, 0, 0), `581.46` = c(0, 
0, 0, 0, 0, 0), `651.71` = c(0, 0, 0, 0, 0, 0), `732.59` = c(0, 
0, 0, 0, 0, 1), `817.56` = c(0, 0, 0, 1, 0, 0), `896.24` = c(0, 
0, 0, 0, 0, 0), `971.77` = c(0, 1, 1, 1, 0, 1), `1038.91` = c(0, 
0, 0, 0, 0, 0), MW = c(3.9, 6.4, 7.4, 8.1, 9, 9.4)), .Names = 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", "MW"), row.names = c("Merc", 
"Peug", "Fera", "Fiat", "Opel", "Volv"
), class = "data.frame")

The plotting code:

## Plotting
meltDF = melt(df, id.vars = 'MW')
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:

Answer 1

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