I've got a strange problem with plotting the fitted values of a glm.
My code is:
Data <- data.frame("Sp" = c(111.4, 185, 231, 272.5, 309, 342, 371, 399,
424, 447, 469, 489, 508, 527, 543, 560, 575, 589, 603, 616, 630, 642, 653,
664, 675, 685, 695, 705, 714, 725, 731, 740), "nrC" = 1:32)
modell <- glm(Sp ~ nrC, data = Data, family = Gamma)
pred <- predict(modell, newdata = data.frame("nrC" = 1:32), type = "response")
plot(Data$nrC, Data$Sp, xlim = c(0, 40), ylim = c(50, 1000))
lines(Data$nrC, pred, col = "blue")
The blue line representing the fitted values seems to be ok, apart from being horizontally mirrored.
I'm relatively new to this, so maybe I'm missing something obvious here, but I can't figure out what's wrong.
Doing the same with the data presented here works perfectly fine.
I'be grateful for any hints!
The gamma distribution isn't quite right for this data set. The data shown in the plot as you have it formulated shows a square root-ish looking function. Try specifying the model like this:
modell <- glm(Sp ~ sqrt(nrC), data = Data, family = gaussian)
pred <- predict(modell, newdata = data.frame("nrC" = 1:32), type = "response")
plot(Data$nrC, Data$Sp, xlim = c(0, 40), ylim = c(50, 1000))
lines(Data$nrC, pred, col = "blue")