# How to Solve R Error: continuous value supplied to discrete scale

by | Programming, R, Tips

This error occurs if you use a numeric variable for the fill, color, and shape aesthetics in ggplot when it expects a factor variable. You can solve this error by using a factor class variable as the grouping variable. You can convert numeric values to `factor` using the `factor()` function.

This tutorial will go through the error in detail and how to solve it with code examples.

## Example

Consider a dataset consisting of 500 random values sampled from the normal distribution. Each of the samples falls into one of five catogories.

```set.seed(0)
df <- data.frame(x = rnorm(500), category= 1:5)

Let’s run the code to see the data frame:

```   x category
1  1.2629543        1
2 -0.3262334        2
3  1.3297993        3
4  1.2724293        4
5  0.4146414        5
6 -1.5399500        1```

Next, we will attempt to create five box plots to show the distribution of the values within the five data groups.

```library("ggplot2")

ggplot(df, aes(x, group = category, fill = category)) +
geom_boxplot() +
scale_fill_manual(values = c("magenta", "steelblue",
"blue", "purple",
"yellow"))```

Let’s run the code to see what happens:

`Error: Continuous value supplied to discrete scale`

The error occurs because the fill argument in the `aes()` function call expects a `factor` variable not `numeric`. We can verify that category is numeric using `is.numeric()`:

`is.numeric(df\$category)`
` TRUE`

### Solution: Use factor() To convert numerical values to factor

We can solve this error by converting the category variable to a factor using the built-in factor function. Let’s look at the revised code:

```library("ggplot2")

ggplot(df, aes(x, group = category, fill = factor(category))) +
geom_boxplot() +
scale_fill_manual(values = c("magenta", "steelblue",
"blue", "purple",
"yellow"))```

Let’s run the code to get the result:

We successfully plotted the five box plots.

## Example #2: Default data set example mtcars and ggplot2

Let’s look at an example of plotting three variables from the mtcars dataset. We will attempt to plot miles-per-gallon (mpg) against weight (wt) with the number of cylinders (cyl) as the colour and shape of the points. We will also plot lines of best fit for the three cyl categories (4, 6, 8).

```ggplot(mtcars, aes(x=wt, y=mpg, color=cyl, shape=cyl)) +
geom_point() +
geom_smooth(method=lm, se=FALSE, fullrange=TRUE)+
scale_shape_manual(values=c(3, 16, 17))+
scale_color_manual(values=c('#999999','#E69F00', '#56B4E9'))+
theme(legend.position="top")```

Let’s run the code to see what happens:

`Error: Continuous value supplied to discrete scale`

The error occurs because the `color` and `shape` arguments in the `aes()` function call need to be factors not numeric.

### Solution

We can solve this error by converting `cyl` from `numeric` to `factor` using the `factor()` function. Let’s look at the revised code:

```ggplot(mtcars, aes(x=wt, y=mpg, color=factor(cyl), shape=factor(cyl))) +
geom_point() +
geom_smooth(method=lm, se=FALSE, fullrange=TRUE)+
scale_shape_manual(values=c(3, 16, 17))+
scale_color_manual(values=c('#999999','#E69F00', '#56B4E9'))+
theme(legend.position="top")```

Let’s run the code to get the result: