multiple time series plots

Dear Friends,

I am trying to add two time series curves into one plot. I don't get any error but the lines are not added to my plot.
I am trying the following code;

1) df <- as.data.frame (read_excel("C:/Users/NO2.xlsx", "trend))

2) plot(df$year, df$Nation, type="o", col="blue", lty=1, ylab="change" )

3) lines(df$year, df$East, col="red",lty=2)

4) lines(df$year, df$center, col="dark red", lty=3)

I need to mention that Nation, East and Center are some probability values.
And year, is a sequence of years. Lines 1 and 2 are run properly but lines 3 and 4 are not added.

I would appreciate any idea.

Regards,

Peri