Sine curve fit using lm and nls in R

Question

I am a beginner in curve fitting and several posts on Stackoverflow really helped me.

I tried to fit a sine curve to my data using lm and nls

Answer 1

This is because the NA values are removed from the data to be fit (and your data has quite a few of them); hence, when you plot fit.lm$fitted the plot method is interpreting the index of that series as the 'x' values to plot it against.

Try this [note how I've changed variable names to prevent conflicts with the functions time and data (read this post)]:

Data <- read.table(file="900days.txt", header=TRUE, sep="")
Time <- Data$time 
temperature <- Data$temperature

xc<-cos(2*pi*Time/366)
xs<-sin(2*pi*Time/366)
fit.lm <- lm(temperature~xc+xs)

# access the fitted series (for plotting)
fit <- fitted(fit.lm)  

# find predictions for original time series
pred <- predict(fit.lm, newdata=data.frame(Time=Time))    

plot(temperature ~ Time, data= Data, xlim=c(1, 900))
lines(fit, col="red")
lines(Time, pred, col="blue")

This gives me:

Which is probably what you were hoping for.