The themes layer is used to style all the non-data ink of the plot, i.e., all the visual elements that are not part of data.

When we create a chart using ggplot2, it automatically uses a default theme. For theming we have a few choices:

1. Use the default theme (What we have seen so far). The default theme is called theme_grey().
2. Use a built-in theme. There are two built-in themes 1) theme_grey() which is default and 2) theme_bw(), a theme with a white background.
3. Modify/override specific elements of the default theme. This is done by using the theme() function. It overrides the graphical parameters of the default theme.
4. Create your own theme

Use the Default Theme

The following chart is created using the default theme:

g <- ggplot(df,aes(x=Duration.of.Credit..in.months.,y=Credit.amount,color=Loan.Quality))
g+geom_point()+geom_smooth()+
  facet_wrap(~Type.of.housing)

Here is the definition of theme_gray theme, straight from ggplot2's source code:

theme_gray <- function(base_size = 12) {
  structure(list(
    axis.line =         theme_blank(),
    axis.text.x =       theme_text(size = base_size * 0.8 , lineheight = 0.9, colour = "grey50", vjust = 1),
    axis.text.y =       theme_text(size = base_size * 0.8, lineheight = 0.9, colour = "grey50", hjust = 1),
    axis.ticks =        theme_segment(colour = "grey50"),
    axis.title.x =      theme_text(size = base_size, vjust = 0.5),
    axis.title.y =      theme_text(size = base_size, angle = 90, vjust = 0.5),
    axis.ticks.length = unit(0.15, "cm"),
    axis.ticks.margin = unit(0.1, "cm"),
    legend.background = theme_rect(colour="white"), 
    legend.key =        theme_rect(fill = "grey95", colour = "white"),
    legend.key.size =   unit(1.2, "lines"),
    legend.text =       theme_text(size = base_size * 0.8),
    legend.title =      theme_text(size = base_size * 0.8, face = "bold", hjust = 0),
    legend.position =   "right",
    panel.background =  theme_rect(fill = "grey90", colour = NA), 
    panel.border =      theme_blank(), 
    panel.grid.major =  theme_line(colour = "white"),
    panel.grid.minor =  theme_line(colour = "grey95", size = 0.25),
    panel.margin =      unit(0.25, "lines"),
    strip.background =  theme_rect(fill = "grey80", colour = NA), 
    strip.text.x =      theme_text(size = base_size * 0.8),
    strip.text.y =      theme_text(size = base_size * 0.8, angle = -90),
    plot.background =   theme_rect(colour = NA, fill = "white"),
    plot.title =        theme_text(size = base_size * 1.2),
    plot.margin =       unit(c(1, 1, 0.5, 0.5), "lines")
  ), class = "options")}

Use a built-in Theme

We can use another built-in theme by adding the theme name to the plot function using a + sign.

The theme_bw() function produces a mostly (but not entirely) black-and-white theme.

g <- ggplot(df,aes(x=Duration.of.Credit..in.months.,y=Credit.amount,color=Loan.Quality))
g+geom_point()+geom_smooth()+facet_wrap(~Type.of.housing)+theme_bw()

Modify/Override specific elements of the chosen theme

We can use the default theme or another built-in theme and then override some elements of it using the theme() function.

The visual elements can be classified as one of the three different types:

• Text
• Line
• Rectangle

Each type can be modified by calling the appropriate function, namely, element_text(), element_line() and element_rect().

Let's take a few examples to understand how this works.

Change Plot Background to Light Yellow

By default, the entire plot has a white background. We will change to to light green color. The background color of the plot is defined by plot.background() in the default function. The background is a part of the rectangle shape, so we can modify it using the element_rect() function. We will change the background color to light yellow and add a gray border of size 2.

g <- ggplot(df,aes(x=Duration.of.Credit..in.months.,y=Credit.amount,color=Loan.Quality))
plot<-g+geom_point()+geom_smooth()+facet_wrap(~Type.of.housing)
plot+theme(plot.background = element_rect(fill = "#FFF6B0",color = "#737373", size = 2))

Change Axis Line and Tick Color

The plot lines can be modified using the element_line() function. Take a look at the default theme function above. The axis lines are axis.line and axis ticks are defined with axis.ticks. Also notice that the panels have a grid which is defined by panel.grid. We will remove the grids by using element_blank().

plot+theme(plot.background = element_rect(fill = "#FFF6B0",color = "#737373", size = 2),
           panel.grid = element_blank(),
           axis.line = element_line(color = "#FF7E42"),
           axis.ticks = element_line(color = "#FF7E42")
           )

Remove Panel and Legend Backgrounds

We will now go a step further and remove the backgrounds of the panels and the legends as shown below:

g <- ggplot(df,aes(x=Duration.of.Credit..in.months.,y=Credit.amount,color=Loan.Quality))
plot<-g+geom_point()+geom_smooth()+facet_wrap(~Type.of.housing)
plot+theme(plot.background = element_rect(fill = "#FFF6B0",color = "#737373", size = 2),
           panel.grid = element_blank(),
           axis.line = element_line(color = "#FF7E42"),
           axis.ticks = element_line(color = "#FF7E42"),
           panel.background = element_blank(),
           legend.key = element_blank(),
           legend.background=element_blank(),
           strip.background = element_blank()
           )

Change Text in the Chart

We can make changes to the various text elements using the element_text() function. Let's make the following changes:

• Change strip text (text that appears in panels) to pink (#E1315B) and increase their size to 16.
• Change color of x-axis and y-axis titles to blue (#008DCB) and make them italics

plot+theme(plot.background = element_rect(fill = "#FFF6B0",color = "#737373", size = 2),
           panel.grid = element_blank(),
           axis.line = element_line(color = "#FF7E42"),
           axis.ticks = element_line(color = "#FF7E42"),
           panel.background = element_blank(),
           legend.key = element_blank(),
           legend.background=element_blank(),
           strip.background = element_blank(),
           strip.text = element_text(size = 16, color = "#E1315B"),
           axis.title.y = element_text(color = "#008DCB", hjust = 0, face = "italic"),
           axis.title.x = element_text(color = "#008DCB", hjust = 0, face = "italic"),
           axis.text = element_text(color = "black")
           )

Change Legend Position

We can modify various aspects of the legend using the theme() function. In the default theme, the legend has the following properties:

legend.background = theme_rect(colour="white"), 
    legend.key =        theme_rect(fill = "grey95", colour = "white"),
    legend.key.size =   unit(1.2, "lines"),
    legend.text =       theme_text(size = base_size * 0.8),
    legend.title =      theme_text(size = base_size * 0.8, face = "bold", hjust = 0),
    legend.position =   "right"

As you can see, the default position is right. The other options are 'bottom', 'left', 'top' or 'none'.

In the following example, we change the legend position to bottom.

plot+theme(plot.background = element_rect(fill = "#FFF6B0",color = "#737373", size = 2),
           panel.grid = element_blank(),
           axis.line = element_line(color = "#FF7E42"),
           axis.ticks = element_line(color = "#FF7E42"),
           panel.background = element_blank(),
           legend.key = element_blank(),
           legend.background=element_blank(),
           strip.background = element_blank(),
           strip.text = element_text(size = 16, color = "#E1315B"),
           axis.title.y = element_text(color = "#008DCB", hjust = 0, face = "italic"),
           axis.title.x = element_text(color = "#008DCB", hjust = 0, face = "italic"),
           axis.text = element_text(color = "black"),
           legend.position = "bottom"
           )

This way we can modify almost every element of the ggplot and achieve any kind of styling we want.