# Trend line not fitting data?

Hey can anyone help me with this trend line? Why is it not fitting the data? Thanks

Fish\$log.Weight <- log10(Fish\$Weight)
Fish\$log.Width <- log10(Fish\$Width)

fish.lm <- lm(log.Weight ~ log.Width, Fish)

# qqplot:

plot(fish.lm, which =2, las=1)

# histogram

hist(resid(fish.lm), main = "Residuals", las=1)

summary(fish.lm)

plot(log.Weight~log.Width, data=Fish, pch=16, las=1,
xlab=expression(paste("log.Width", (cm))),
ylab="log.Weight (g)", log="xy")

#add the regression line from the fitted model:
abline(fish.lm, col='blue') Using the axes to a log scale seems to be the problem. Here are some plots with data I invented.

``````DF <- data.frame(Width=2:50)
DF\$Weight <- DF\$Width*4.32
DF\$log.Weight <- log10(DF\$Weight)
DF\$log.Width <- log10(DF\$Width)
FIT <- lm(log.Weight~log.Width,data=DF)
FIT2 <- lm(log10(log.Weight)~log10(log.Width),data=DF)

plot(log.Weight~log.Width,data=DF,log="xy")
abline(FIT)
abline(FIT2,col="red")
`````` ``````plot(log.Weight~log.Width,data=DF)
abline(FIT)
`````` Created on 2022-06-10 by the reprex package (v2.0.1)

Thanks for your help! just with FIT2 - am I then taking a log of a log?
How would I predict values of weight with this line?

The solution here is to use untf=TRUE for abline param when you are using the log="xy" scale.

If untf is true, and one or both axes are log-transformed, then a curve is drawn corresponding to a line in original coordinates, otherwise a line is drawn in the transformed coordinate system. The h and v parameters always refer to original coordinates.

``````plot(log.Weight~log.Width,data=DF,log="xy")
abline(FIT,col="blue",untf = TRUE)``````
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