The theme function should not be inside of the labs() function. At least, I have never seen that and I would expect it to throw an error. I may just be unaware of that method, but try putting theme() after the labs(), connecting them with a + sign.
Also, it looks like your dates are being read as characters, not as numeric dates. There are convenient functions in the lubridate package. If your dates have the format m-d-y, use the mdy() function.