Problems plotting time-series with multiple lines

Dear Sirs,

I want to plot a time series with lines, one for each variable.
I have a dataset with dates, and the values.
How can i plot?
I could plot one variable using index plot, bu i want to put the labels on X
axis. But i had two problems:
1) The plot function, when i try to plot(x,y), incorectly sort the date (on
X axis). My dataset has the date in string format "%d/%m/%Y).
If i try to converto to date using
as.Date(Dataset.dates,"%d/%m/%Y)
it interprets the %Y incorrectly.

2) I have 187 rows. So i have to plot only some of the dates, not all.