ggplot2 没有 plot 所有点

Question

I am trying to make a graph in ggplot2 of 7 points in the shape of a circle, but trying to graph them only shows me 6 and I don't know why this happens.

The code is the following:

# Function for the points
circleFun <- function(center = c(-1, 1), diameter = 1, npoints = 7) {
  r <- diameter / 2
  tt <- seq(0, 2 * pi, length.out = npoints)
  xx <- center[1] + r * cos(tt)
  yy <- center[2] + r * sin(tt)
  return(data.frame(x = xx, y = yy))
}

# example with 7 points
ej <- 
  circleFun(diameter = 50, center = c(50,50), npoints = 7)


# plot

ej |> 
  ggplot(aes(x = x, y = y)) +
  geom_point(alpha = 0.4) +
  theme_bw()

Does anyone know why this happens?

Answer 1

Rows 1 and 7 are identical, so their points are overlapped. The dot is a bit darker (per your alpha = 0.4). You can make this obvious by adding x = jitter(x) (For demonstration, not that you'd do that in production). I'm not sure what you're expecting to see given the identical data.

If you want 7 distinct points, then I suggest you create n+1 and remove the last (or first) point.

circleFun <- function(center = c(-1, 1), diameter = 1, npoints = 7) {
  r <- diameter / 2
  tt <- seq(0, 2 * pi, length.out = npoints + 1)  # changed
  xx <- center[1] + r * cos(tt)
  yy <- center[2] + r * sin(tt)
  data.frame(x = xx, y = yy)[-1,,drop = FALSE]    # changed
}

## unchanged from here on
ej <- 
  circleFun(diameter = 50, center = c(50,50), npoints = 7)
ej |> 
  ggplot(aes(x = x, y = y)) +
  geom_point(alpha = 0.4) +
  theme_bw()

(BTW, there is no need for an explicit call to return(.) , especially when it is the only end-point of the function and "obvious" based on data flow. It certainly doesn't hurt, but it adds one step on the call stack that adds no value. It may be declarative/self-documenting, and as such this is a style/subjective point.)