Understanding the Issue with Values of Wrong Group in RStudio

In this article, we will delve into a common issue faced by R users, particularly those using RStudio. The problem revolves around the incorrect usage of values from the wrong group when generating plots within data.table().

Introduction to Data.Table and Plot()

data.table() is a popular data manipulation library in R that offers efficient data structures for big data analytics. One of its key features is the ability to perform operations on grouped data, which can be achieved through the use of the by argument.

When it comes to plotting data with plot(), we often need to specify various parameters such as the x and y limits (i.e., ylim). However, when working within a data.table() environment, things get complicated due to differences in how RStudio handles graphics devices compared to the standard console or GUI.

The Problem and Its Causes

The problem arises when we try to plot data from different groups using plot(). In this case, we observe that the values of one group are being used instead of the intended group. This issue can be observed in several examples, including:

library(data.table)
set.seed(23)
Example <- data.table('group' = rep(c('a', 'b'), each = 5), 'value' = runif(10))
layout(1:2)
par('mai' = rep(.5, 4))

Example[, plot(value, ylim = c(0, 1)), by = group] # Example 1

When executed in the standard R console or GUI, the correct values are used. However, when run in RStudio, the values of the second group ('b') are mistakenly used for both plots.

Debugging and Finding the Cause

To understand why this issue occurs, we need to delve into the graphics devices used by RStudio and the standard console/GUI. After debugging graphics:::plot.default, we discovered that it calls different graphics devices:

• Quartz (RStudio)
• X11 (standard console/GUI)

We also found that a specific function called plot.xy() is responsible for this issue.

Solution and Workarounds

To resolve the problem, we can try several approaches:

1. Using a Different Graphics Device

One solution is to use the Quartz graphics device by calling the quartz() function before running our plotting code:

# on RstudioGD
quartz()
Example[, plot(value, ylim = c(0, 1)), by = group] # Example 1

This can help avoid issues with the wrong group values.

2. Explicitly Copying Values

Another solution is to explicitly copy the value column when plotting:

Example[, plot(copy(value), ylim = c(0, 1)), by = group] # Example 1

By doing so, we can ensure that each group’s values are used separately.

Conclusion

In this article, we have explored a common issue with plotting data from different groups in RStudio. We have identified the causes of the problem and discussed potential solutions. By understanding how graphics devices work and using explicit copying or switching to a different graphics device, users can resolve issues with incorrect group values when generating plots within data.table().

Additional Insights

• This issue is likely due to recent changes in Rv3.1+ that shallow copy function arguments rather than deep copying.
• The problem might be related to the way subgroups are handled in data.table and dplyr.
• Users can verify this by doing debug(graphics:::plot.default) before running their plotting code.

By following these guidelines, you should be able to resolve issues with plotting values from wrong groups when working within RStudio.

Last modified on 2023-09-04