What should be the correct frequency of daily data?

I have a time series which represents the amount of a certain product sold throughout the year 2018 (from 2018/01/01 to 2018/12/31); is it correct to think of a frequency of 7 observations per cycle? and if so, what is my cycle? one week? I try to understand this in order to decompose my time series avoiding the error Error in decompose(tsData) : time series has no or less than 2 periods. This is my R script and my data.

library(forecast)
library(sweep)
library(timetk)

Data <- read.delim("R Project/Dataset/MyData.txt")
DataFrame <- data.frame(Data, 
                        Date = seq(as.Date("2018-01-01"), as.Date("2018-12-31"), 
                                   by = "day"))
inds <- seq(as.Date("2018-01-01"), as.Date("2018-12-31"), by = "day")
tsData <- ts(Data, start = c(2018, as.numeric(format(inds[1], "%j"))), 
                   frequency = 365)
print(tsData)
plot(tsData)
Axis(inds, side = 1, at = seq(inds[1], tail(inds, 1) + 60,
           by = "1 months"), format = "%b %Y")
comp = decompose(tsData)
#comp = stl(tsData)
plot(comp)
fit <- auto.arima(tsData)
fore <- forecast(fit, h = 15, level = 99.5)
plot(fore, xaxt = "n")
Axis(inds, side = 1, at = seq(inds[1], tail(inds, 1) + 60, by = "1 months"), 
                     format = "%b %Y")

This is MyData.txt file

Solution

The frequency = parameter in ts() function indicates the number of observations before pattern repetition. If you set a seasonality of 365 (1 year) with 1 year of data it will have only 1 period and so decompose() tells you: time series has no or less than 2 periods. As you said "7 observations per cycle", you may want to set frequency equal to 7. Or if you want to analyze year seasonality put more data in tsData.

Just change:

# ....
tsData <- ts(Data, start = c(2018, as.numeric(format(inds[1], "%j"))), frequency = 365)
# ...

to :

# ...
### weekly seasonality
tsData <- ts(Data, start = c(2018, as.numeric(format(inds[1], "%j"))), frequency = 7)
#...

and now decompose works:

comp = decompose(tsData) # NO ERROR
### get the plot
plot(comp)
# ... rest of your code ...

here the plot:

EDIT on your comment:

The X-axis on the plot depends on how you declare the start, please have a look at ts documentation.

If you want to have the 2018 year value you can simply use (see documentation) autoplot() :

# ... rest of code ...
autoplot(tsData)
# ... rest of code ...

that is also highly customizable, if you want to know how to customize the plot (made through ggplot2 package) just have a look at documentation and all the posts on this blog etc.