Changing the shape of one point or few points in a scatter plot in R

I have a set of points in a scatter plot as below. I want to change the shape of one point or few points. I searched for this but could not find a way to do it.

I want to achieve like this

And like this

Code:

df <- data.frame(x = c(1,2,2,3,3.5,4,4.5,5,5.5,6,1.5,2,2,2,2,1.5,2.5,3,3,3,3,5.5,5,6,5.5,7)
                 ,y = c(2,1,2,2,2,2,2,2,1.5,2,2.5,3,3.5,4,4.5,3.5,3.5,2,3,3.5,4,2.5,3,3,4,3.5))

library(ggplot2)
library(extrafont)

# helper dataframe for axis
df_arrow <- data.frame(x = c(0, 0),
                       y = c(0, 0),
                       xend = c(0, 8),
                       yend = c(8, 0)) 


ggplot(df,aes(x, y)) + 
  geom_point(colour = "blue", size = 5, shape = 3)+
  scale_x_continuous(breaks = 1:7, expand = expansion(add = c(0, 1)))+
  scale_y_continuous(breaks = 1:7, expand = expansion(add = c(0, 1)))+
  coord_fixed(xlim = c(0, 7), ylim = c(0, 7), clip = "off")+
  geom_segment(data = df_arrow, aes(x = x, xend = xend, y = y, yend = yend), size = 0.75, colour = "black",
               arrow = arrow(angle = 20, length = unit(3, "mm"), ends = "last", type = "closed"), linejoin = "mitre") +
  annotate("text", x = c(7.8, 0.3), y = c(0.3, 7.8), label = c("italic(x)", "italic(y)"), parse = TRUE, size = 6,  family = "Times New Roman")+
  labs(x = NULL,
       y = NULL)+
  theme_bw()+
  theme(panel.grid.major = element_line(colour = "gray80"),
        panel.grid.minor = element_blank(),
        panel.border = element_blank(),
        axis.ticks.length = unit(1, "mm"),
        text = element_text(size = 18,  family = "Times New Roman"))

How is this?

Create a new column using dplyr::mutate, which is conditional upon the x-coordinates (for example, but it could be anything). Then, use this column within aes to control the shape size.

Also, you can use scale_shape_manual and scale_colour_manual to manually control the shape and colours. It's not clear to me what shape you want but you would just need to change the arguments in scale_shape_manual.

EDIT:

Since you specifically need a different symbol, then you need to use geom_text instead.

df %>% 
  dplyr::mutate(z = ifelse(x >= 5, "-", "+")) %>%
  ggplot(aes(x, y)) +
  geom_text(size = 12, aes(colour=z, label=z)) +
  scale_x_continuous(breaks = 1:7, expand = expansion(add = c(0, 1)))+
  scale_y_continuous(breaks = 1:7, expand = expansion(add = c(0, 1)))+
  coord_fixed(xlim = c(0, 7), ylim = c(0, 7), clip = "off")+
  geom_segment(data = df_arrow, aes(x = x, xend = xend, y = y, yend = yend), size = 0.75, colour = "black",
               arrow = arrow(angle = 20, length = unit(3, "mm"), ends = "last", type = "closed"), linejoin = "mitre") +
  annotate("text", x = c(7.8, 0.3), y = c(0.3, 7.8), label = c("italic(x)", "italic(y)"), parse = TRUE, size = 6,  family = "Times New Roman")+
  labs(x = NULL,
       y = NULL)+
  theme_bw()+
  theme(panel.grid.major = element_line(colour = "gray80"),
        panel.grid.minor = element_blank(),
        panel.border = element_blank(),
        axis.ticks.length = unit(1, "mm"),
        text = element_text(size = 18,  family = "Times New Roman")) +
  scale_shape_manual(values=c(8, 9)) +
  scale_colour_manual(values = c('red', 'blue'))