r - x axis label in plot overlaps -

I describe a time series graph of search volume on the word "shopping", from January 2004 to December Deseasonalized during 2014. Although the date of my X axis shows all my months for my data and it overlaps, which looks messy. How can I change it only in those years?

This is my code:

  ggplot (data = merge data, AES (x = date, y = shoppingdesage, group = 1)) + geom_line (color = "red + "Excach" (size = 1) + xlab ("date [year]") + elab ("shopping dozen [search volume]") + ggtitle ("search volume on shopping from 2004 to 2014") + theme (axis.text .x = element_text (size = 20, angle = 90, face = "bold"), axis. Text.y = element_text (size = 20, face = "bold"), axis-tithle = element_text (size = 22, faces Try using "bold":  
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