Contour plot not fully filled with ggplot

Here is an example:

Generate coordinates:

b = c(0, 5, 10, 15, 20)
a = (1:30)/10

Generate all combinations of coordinates

df <- expand.grid(a, b)

Generate c via tcrossprod of a and b + 1 (this is completely arbitrary, but generates a nice pattern)

df$c <- as.vector(a %o% (b+1))

ggplot(df, aes(x = Var1, y = Var2, z = c, fill = c)) +
  geom_raster(interpolate = T) + #interpolate for success 
  scale_fill_gradientn(colours = rainbow(10))

Generally, if you want to plot a matrix of values ​​(z-values) in ggplot, you need to convert it to long format via meltin reshape2or collect in tidyr, and then use it for plotting.

Your data is very sparse, one workaround is to generate missing data. I'll show how to do it with the loess function:

model <- loess(c ~ a + b, data = DF) #make a loess model based on the data provided (data in OP)

z <- predict(model, newdata = expand.grid(a = (10:30)/10, b = (0:200)/10)) #predict on the grid data

df <- data.frame(expand.grid(a = (10:30)/10, b = (0:200)/10), c = as.vector(z)) #append z to grid data

ggplot(df, aes(x = a, y = b, z = c, fill = c)) +
  geom_raster(interpolate = T)+
  scale_fill_gradientn(colours = rainbow(10))