ggplot() + geom_line(data = visits_1, aes(month, visits, group = 1,colour = "#008FD5", size=2)) + facet_wrap(~visits_1$year) + geom_line(data = x, aes(Month, Visits))

str(visits_1)
'data.frame':	164 obs. of  6 variables:
 $ museum: chr  "NATIONAL GALLERY" "NATIONAL GALLERY" "NATIONAL GALLERY" "NATIONAL GALLERY" ...
 $ year  : chr  "2004" "2004" "2004" "2004" ...
 $ month : chr  "4" "5" "6" "7" ...
 $ visits: num  437000 391000 319000 460000 443000 325000 425000 448000 410000 402000 ...
 $ season: chr  "" "" "" "" ...
 $ date  : Date, format: "2004-04-01" "2004-05-01" "2004-06-01" "2004-07-01" ...
 - attr(*, "na.action")=Class 'omit'  Named int [1:5] 2857 2858 2859 2860 2861
  .. ..- attr(*, "names")= chr [1:5] "2857" "2858" "2859" "2860" ...
>

> str(x)
'data.frame':	12 obs. of  2 variables:
 $ Month : chr  "1" "10" "11" "12" ...
 $ Visits: num  410975 419828 418250 421863 407462 ...