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I'm doing a plot with ggplot2 but when I add the function geom_smooth with method = loess, my graph doesn't work . Indeed, it creates curves that don't match with data. When I change this line in my script, by deleting it , or using another method , graphs work correctly.

I already checked that my data are numeric and that there are not extrem values because of a missing decimal, but it is not that.

How can I correct that to make my geom_smooth work with the loess method ?

Nathan FRIZOT
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  • Please make your question reproducible by including some code that produces sample data. For instance you could run `dput(donnees_tot_g` and paste the output into the body of your question. – Jon Spring Jul 05 '22 at 15:45
  • ... and please provide the code for `method="loess"`, the case that doesn't work, as well as the code for `method="lm"`, the case that does. – Limey Jul 05 '22 at 16:28
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    My guess is that you're confusing `geom_smooth(method="loess")` doesn't work (i.e., produces a wrong answer) with `geom_smooth(method="loess")` doesn't produce what I hoped it would. It could "work", but not produce the answer you were looking for. – DaveArmstrong Jul 05 '22 at 16:35

2 Answers2

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Your issue is that the default parameters of the loess are not working well for your dataset. You have only a small number of discrete x values so it doesn't know how best to fit it. For example, if you look at the default value of span in base::loess() (which ggplot2::geom_smooth(method = "loess") calls under the hood) you can find that the default value is span = 0.75. If you just increase to span = 0.8 you get what I assume is closer to what you wanted. For more on the span parameter you can see this answer.

library(tidyverse)

d %>% 
  ggplot(aes(x = quantity, y = fecundity, col = color)) +
  geom_jitter(size = 3) +
  geom_smooth(method = "loess", span = 0.8, alpha = 0.2) +
  scale_x_continuous(breaks=c(0.1,0.3,0.6,0.9,1.5), limits=c(0.1,1.5))+
  scale_colour_manual(values=c("20S" = "aquamarine1","25S" = "aquamarine3","28S" =
                                 "aquamarine4","20Y" = "darkgoldenrod1","25Y" = "darkgoldenrod3", "28Y" = "darkgoldenrod4"))+
  ggtitle("Fécondité en fonction du traitement de nourriture et de la température")+
  xlab("Quantité nutritionnelle") + ylab("Fécondité (nb d'oeufs/femelle)")+
  theme_grey(base_size = 22)

Created on 2022-07-05 by the reprex package (v2.0.1)

Dan Adams
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You need to quote the method type, e.g. use geom_smooth(method="loess")

mhovd
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