Using a for-loop of characters to plot several lines with specific colors

In data analysis and visualization, it is common to have multiple datasets that need to be plotted on the same graph. When dealing with categorical variables, such as basin names, we often want to color-code each line based on its corresponding category.

Problem Description

The problem presented in the question revolves around plotting multiple lines on a single graph, where each line represents a subset of data grouped by the characters in column ‘basin’. The original solution uses a for-loop, but it has an issue: all lines are colored blue and solid. We want to change this behavior so that each line is assigned a specific color.

Background

To understand why the provided code does not work as expected, we need to delve into how the lines() function works in R. The lines() function is used to plot multiple lines on a single graph. It takes three arguments: the first is an expression that defines the y-values of each line; the second is an expression that defines the x-values of each line; and the third is a list of arguments that customize the appearance of the lines.

In our case, we want to use a for-loop to iterate over the basin names and plot corresponding lines on the graph. The key insight here is that when using lines(), we need to access both the index of the vector and the variable itself within each iteration.

Solution

To solve this problem, we can use a simple modification to the original code. We’ll replace the hardcoded colors with a color cycling scheme based on the basin names.

env <- data.frame(basin = c('BLK','DUC','WHP','BLK','DUC','WHP','BLK','DUC','WHP'),
                  sal = c(5,6,3,2,4,5,6,8,4),
                  date = c(2013,2013,2013,2015,2015,2015,2017,2017,2017))

basinlist <- c('BLK','DUC','WHP')

plot(sal~date, data = env, type = 'n', ylim = c(0,10), ylab = 'Salinity')

for (ii in seq_along(basinlist)) {
  i <- basinlist[ii]
  colors <- c('red', 'green', 'blue') # example color palette
  
  # assign a specific line style based on the character
  styles <- c(1,1,2)
  
  lines(sal[basin==i] ~ date[basin==i], data = env,
        col = colors[ii],
        lty = styles[ii])
}

Explanation

The key idea here is to use a vector of colors that corresponds to the basin names. We can do this by creating a list of colors (colors) and indexing into it using the current basin index (ii). This way, each line will be assigned a specific color based on its corresponding basin name.

Additionally, we want to assign a specific line style based on the character in the basin name. For example, we might want to make certain lines thicker or dashed. We can achieve this by creating another vector of styles (styles) and indexing into it using ii. This way, each line will have a unique appearance that corresponds to its corresponding basin name.

Advice for Further Improvement

While the provided solution works well for this specific problem, there are several ways to improve it:

• Color Palette: Instead of hardcoding a color palette, consider loading a set of predefined colors from a library like RColorBrewer or using an external source like matplotlib.colors.
• Line Styles: Consider adding more complexity to the line styles by incorporating additional parameters, such as thickness or dash patterns.
• Grouping: If you have multiple datasets with different characteristics, consider grouping them together based on their categorical variables.

By applying these suggestions, you can create a more flexible and customizable solution that suits your specific data visualization needs.

Last modified on 2024-12-02